Tips on brake bleeding please

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Offline A Price

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Tips on brake bleeding please
- Jan 09, 2022 - 15:31
Have spent all afternoon trying to bleed the brakes and getting nowhere.  Mostly new pipework, new fexi hoses and a new master cylinder fitted. Tried bleeding and got a soft pedal. Have tried an Ezi bleed kit and getting fluid to all four corners but pedal still not firm. Been round all unions and everything appears tight. Does the angle I have the landy jacked make a difference? Any suggestions as I am stumped Thanks everybody

Offline antarmike

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Re: Tips on brake bleeding please
Reply #1 - Jan 09, 2022 - 15:42
Have spent all afternoon trying to bleed the brakes and getting nowhere.  Mostly new pipework, new fexi hoses and a new master cylinder fitted. Tried bleeding and got a soft pedal. Have tried an Ezi bleed kit and getting fluid to all four corners but pedal still not firm. Been round all unions and everything appears tight. Does the angle I have the landy jacked make a difference? Any suggestions as I am stumped Thanks everybody
Not in my experience. I never have trouble bleeding Land-Rover brakes using an easibleed (at about 20 psi. Chassis level, brakes adjusted up so wheels just turn easily. 

I have never had to adjust brakes to the point of locking them before bleeding, or raising one or other end of the chassis. 

The secret for me is simply to bleed though lots and lots of fluid.  I will run through 1/2 an Easi bleed bottle (or more) at each wheel station,  Long after you think air has stopped exiting that bleeder and fluid is running bubble free, more air miraculously appears from nowhere.

Start with the wheel station furthest from the master cylinder, and move on to the next closest, ending with the wheel closed to the master cylinder.

Offline autorover1

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Re: Tips on brake bleeding please
Reply #2 - Jan 09, 2022 - 16:25
On my S1 I just let the fluid run out under gravity , keeping the reservoir topped up . You really need the brake master cylinder to be nominally level for it to get all the air out. Are you sure its not the shoes that are springing, until they bed in properly, as opposed to  air in the system ?  I changed the fronts on my 86" last year and until they had bedded in  I had a slightly springy pedal which got firmer over the next 100 to 200 miles. I deliberately drove round the Welsh Marches , on  minor roads, with lots of short periods of braking on bends and hills to bed them in. 

Offline nickuk

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Re: Tips on brake bleeding please
Reply #3 - Jan 09, 2022 - 17:10
This topic has been discussed many, many times without any clear resolution of the issue.

Some 80's just don't seem to bleed easily. Extensive discussion does not reveal why!

Pressure bleeding with pressure over the brake reservoir seems to work - modern suction bleeding has been said to work by some but has also been said not to be good.

I have used a Schrader valve soldered into an old reservoir cap and now have a plastic cap that was bought with a bleeding kit.

One caveat: a very rusty and thinned brake lever can spring and feel like air even when the hydraulics are hard and perfect.

Nick.



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Offline davep10000

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Re: Tips on brake bleeding please
Reply #4 - Jan 09, 2022 - 17:25
Bleeding the brakes on my 86 was one of the most tedious jobs - even after following all the advice from previous searches, I could completely change one rear wheel bearing quicker than bleeding the brakes!
I am now with autorover - in that I attach a bleed pipe to each nipple, and open to allow a very slow drip (once every 10 seconds maybe), then go off for a couple of hours or more (checking reservoir now and again).
Hope this helps
Dave.

Offline fulltilt

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Re: Tips on brake bleeding please
Reply #5 - Jan 09, 2022 - 17:35
I use Gunsons Ezi-bleed , try to stick to stated 20 psi , sometimes increase to 30 psi with some difficult systems / some ABS (some ABS - don't drain fluid and switch on the ignition, you may need main-dealer equipment to resolve).  I recently had a problem on a Mondeo , not wishing to place a car tyre on top of engine - I used a barrow wheel as air source.  Went well for a while , then I checked the tube pressure - flat , you do need a good volume tyre/tube.   I try and use the bottle as air-receiver but you have to pay close attention to MC reservoir level.   Have extra cap kit but still at times purchased a proper MC cap & modified for fitting.

Offline fifty seven

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Re: Tips on brake bleeding please
Reply #6 - Jan 09, 2022 - 17:51
forum search will now produce 2 pages listing many previous attempts to explain the same various methods. The same advice repeats time after time.

I use a plastic bag of air over the top of the reservoir fastened with a rubber band cut from old inner tube. I apply steady hand pressure or a lump of metal  to the bag, to bleed each wheel cylinder...furthest away one first . Topping up the reservoir each time is important before moving on . Always  works for me.

Try "Brake bleeding" in forum search  ( If you do ..you may look forwards to enjoying a busy evening  :tic:).
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Offline seriesonenut

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Re: Tips on brake bleeding please
Reply #7 - Jan 09, 2022 - 18:01
I finally sorted mine today but it was hard work!. I am going to try the plastic bag method next time!

fuv3 by Richard Holmes, on Flickr

Offline antarmike

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Re: Tips on brake bleeding please
Reply #8 - Jan 09, 2022 - 18:09
This topic has been discussed many, many times without any clear resolution of the issue.

Some 80's just don't seem to bleed easily. Extensive discussion does not reveal why!

Pressure bleeding with pressure over the brake reservoir seems to work - modern suction bleeding has been said to work by some but has also been said not to be good.

I have used a Schrader valve soldered into an old reservoir cap and now have a plastic cap that was bought with a bleeding kit.

One caveat: a very rusty and thinned brake lever can spring and feel like air even when the hydraulics are hard and perfect.

Nick.

Let me explain why I personally believe pressure bleeding is better than vacuum bleeding.

Air is forced from the master cylinder if there is greater pressure at the master cylinder, than there is at the bleed screw.

This can be achieved in two ways....

a) attaching a pressure bleeder such as easibleed to the master cylinder, and leaving the bleed screw at atmospheric pressure...
or....
b) leaving atmospheric pressure at the master cylinder and reducing the pressure at the bleed screw by applying a vacuum.

The maximum pressure differential obtainable by vacuum bleeding (b) is around 14.7 PSI (or absolute vacuum) and this is only achievable using very sophisticated vacuum pumps.   Commercial vacuum bleeders built to a budget are not going to achieve absolute vacuum, so realistically this limits the pressure differential between Atmospheric pressure  at the master cylinder, and the applied vacuum at the bleed screw to around 14 psi.

The maximum pressure differential achievable sticking to Easibleeds recommended pressure (a) is 20 psi, but many have pushed the safety factor of the pressure vessel / fluid reservoir raising the applied pressure to 25 or even 30 psi.

The science says that if you double the pressure differential (which you can do by using pressure bleeding at 28 psi, rather than vacuum bleeding at -14 psi) then you will vastly increase the flow rate of the fluid through the sytem, boosting its chances of carrying air bubbles out of the system.

I suppose you could do both.   Attach a vacuum bleeder to the bleed screw and a pressure bleeder to the master cylinder at the same time to get something like a pressure differential of 34 psi, but I have always found that simply using around 25 psi in an easibleed give sufficient flow, in terms of volume and speed, to clear the bubbles out of the system.

Note:  On a Health and Safety point, I am not suggesting putting 25 or 30 psi on an easibleeder where recommended pressure should be a max of 20 psi.  It is what I do, however, others would have to make that decision off their own backs, and take whatever extra safety precautions they thought were necessary.

But even using the recommended 20 psi, this is a 33% increase in pressure differential over and above what is feasible when vacuum bleeding.

That is why I pressure bleed.



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Offline Ivor M

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Re: Tips on brake bleeding please
Reply #9 - Jan 09, 2022 - 19:09
The advantage of vacuum bleeding is that any bubble is expanded by the vacuum and moves along the system to the low pressure point . Pressure bleeding compresses the bubble so it moves less. Effectively just a vacuum of 7psi has the same differential pressure as 29psi by pressure e.g. the bubble is twice the size or half the size under pressure. 
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Offline fraserfountain1

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Re: Tips on brake bleeding please
Reply #10 - Jan 09, 2022 - 19:12
I spent a long time with issues bleeding my brakes, I found that putting a pressure bleeder on the reservoir and then pumping the brakes 10-20 times really vigorously and hard was the only way to get the trapped air out. I blocked the front and rear flexis off which did nothing and I believe the problem was occurring somewhere in the master cylinder.

Offline fraserfountain1

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Re: Tips on brake bleeding please
Reply #11 - Jan 09, 2022 - 19:14
I have to add that I never tried a vacuum bleeder.

Offline fulltilt

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Re: Tips on brake bleeding please
Reply #12 - Jan 09, 2022 - 19:38
On a system I know is easy to bleed - quite simply I use the biggest plastic syringe off Ebay , about 1" dia with some tight fitting plastic pipe between syringe & nipple, I don't know what the vacuum created is but a strong assistant on the syringe is best   ;D

Offline Ivor M

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Re: Tips on brake bleeding please
Reply #13 - Jan 09, 2022 - 21:32
Don't forget about a bit of PTFE tape around the bleed screw.

Offline A Price

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Re: Tips on brake bleeding please
Reply #14 - Jan 09, 2022 - 22:32
Thanks for all your tips will have another go tomorrow

Offline TDC

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Re: Tips on brake bleeding please
Reply #15 - Jan 09, 2022 - 23:13

One caveat: a very rusty and thinned brake lever can spring and feel like air even when the hydraulics are hard and perfect.

Nick.

New part too long overdue methinks!

86" etc pedal levers are badly designed. The underside channel collects mud, water and salt directly off the wheel. There is a perfect pocket for this corrosive gunge  at the base of the lever around the pivot bush (max torque point). I have seen many brake levers with big holes hidden in this location, sometimes repaired with weld. I suggest everyone gets out and gives their levers a hit with a hammer before finding out it's rotten in an emergency!     Never seen an MOT person clouting it to check.

Offline antarmike

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Re: Tips on brake bleeding please
Reply #16 - Jan 10, 2022 - 07:42
New part too long overdue methinks!

86" etc pedal levers are badly designed. The underside channel collects mud, water and salt directly off the wheel. There is a perfect pocket for this corrosive gunge  at the base of the lever around the pivot bush (max torque point). I have seen many brake levers with big holes hidden in this location, sometimes repaired with weld. I suggest everyone gets out and gives their levers a hit with a hammer before finding out it's rotten in an emergency!     Never seen an MOT person clouting it to check.

The MOT test has rules that prevenet the tester "clouting" things with a "hammer"

Guidance is given as to just how an MOT is to be carried out in terms of assessing corrosion.

Generally the testing for corrosion has to be done like this....

"Identify the important load bearing members and ‘prescribed areas’ on a vehicle, then check if they are excessively corroded by:
Visual inspection
Use finger and thumb pressure to assess the extent of the corrosion
If necessary, carefully scrape or lightly tap the affected areas with the corrosion assessment tool
Use of the corrosion assessment tool must be restricted to ascertaining that the failure criteria are met and not used for heavy scraping or poking of the affected areas."

That is to say light tapping only with the prescribed tool is permitted.....

https://www.prosol.co.uk/product/mot-approved-hammer-corrosion-assement-tool/

If the brake pedal arm is considerd as a "highly stresed item"

Then this inspection routine is to be followed....

"The severity of corrosion in highly stressed components, such as steering and suspension arms, rods and levers, can be assessed by lightly tapping or scraping with the corrosion assessment tool.

In places that cannot be reached by the corrosion assessment tool, an alternative blunt instrument may be used.

A highly stressed component should be rejected if corrosion has resulted in serious reduction in the overall thickness of the material or has caused a hole or split.

Welded repairs to highly stressed components are not normally acceptable, other than where the component is made up of sections that are welded together. To pass, the repair should appear to be as strong as the original design."



Offline msm80

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Re: Tips on brake bleeding please
Reply #17 - Jan 10, 2022 - 08:47
The advantage of vacuum bleeding is that any bubble is expanded by the vacuum and moves along the system to the low pressure point . Pressure bleeding compresses the bubble so it moves less. Effectively just a vacuum of 7psi has the same differential pressure as 29psi by pressure e.g. the bubble is twice the size or half the size under pressure.

Very good point Ivor. Pressure or vacuum BUT, through back bleeding was what I was always advocating in the many posts we've all contributed too on this topic. I think there is enough on here to write a 'bleeding' book on the subject!! Simple physics, air goes up in a fluid sinks down does it not. Why even struggle sucking air down in a fluid filled pipe when it wants to go up!! On production lines they tend to fill from the slaves I was advised and most of the trade back bleed every time. This if course assumes all components are scrupulously clean to start with and wetted prior to assembly which helps!

I agree with Fultilt a large syringe is brilliant and simple to use requiring only a spanner for the nipple and a short length of rubber tubing which I then connect to another 1m length of aquarium tubing to track air exiting the system. As a precaution on newly rebuilt systems, like Ivor I ensure the nipples are screwed in with ptf tape to make them an air tight fit. If filling from new all round then fill from the slaves first, otherwise on a filled system or where you've disconnected a branch, top up the reservoir first, then using the syringe from each wheel, draw fluid through in turn from the reservoir going round each filled spur in the recommended sequence sucking the fluid down and topping up the fluid as you fill each spur/wheel cylinder each time.

It works every time, provided the master cylinder has been correctly set up to allow fluid to pass through the recuperative seal in the piston and all the brake shoes are correctly set to start with. No massive pumping at the pedal or high pressure Easibleeds (I hate these on older reservoirs as the caps are always blowing off) thus avoiding catastrophic leakages all over your nice paintwork!. The bigger the syringe the better and Vets supplies are good! On a rebuild I tend to get fluid in, then after the first round, check for any leaks before trying the pedal to move all the pistons and dislodge and stubborn bubbles. Then do it all once more, again sucking out fluid will draw any bubbles down as Ivor suggests.

Malcolm

Offline antarmike

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Re: Tips on brake bleeding please
Reply #18 - Jan 10, 2022 - 09:14
The advantage of vacuum bleeding is that any bubble is expanded by the vacuum and moves along the system to the low pressure point . Pressure bleeding compresses the bubble so it moves less. Effectively just a vacuum of 7psi has the same differential pressure as 29psi by pressure e.g. the bubble is twice the size or half the size under pressure.

Surely there will be a pressure gradient along the pipe?  Pressure is constant throughout a closed hydraulic system, but that is not the case for an open system, where the pressure depends on the ease of movement along the system, (Friction, restrictions etc).

Pressure  gradually dropping towards the open end of the system were the pressure returns to atmosheric pressure.

When vacuum bleading  a bubble near the reservoir and master cylinder will be subjct to almost atmospheric pressure and they will not be expanded by the vacuum.

Likewise when pressure on the  bubble reduces as move along the pressure gradient, as they near the brake slave cylinder, they experience almost atmospheric pressure so they will hardly be compressed at all.

Overall I doubt whether expansion and contraction of bubbles (which will be of varying effect along the pipe) can make up for a pressure difference of maximum 14 psi (realistically quite a lot less) compered to somewhere between 20 and 30 psi pressure difference when pressure bleeding. The difference in size of bubbles will be nowhere near as great as you suggest.

I would argue that the bubble are carried by the flow of fluid, and pressure bleeding must always give a greater (faster) flow along the pipes.  The speed of movement of fluid along the pipe in pressure bleeding is what gives it a big advantage over vacuum bleeding.

If the flow is fast enough, the fluid is in effect a piston filling the pipe to its full diameter, pushing everything in front of it forward, be that big bubbles or small bubbles.

A trickle of water along a sewer by only turning on the basin tap will only fill the bottom of the sewage pipe, leaving air above but flush the loo and send a large volume of water , quickly into the sewer is more likely to completely fill the sewage pipe.

Flow rate is everything.



Offline jonhutchings

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Re: Tips on brake bleeding please
Reply #19 - Jan 10, 2022 - 09:47
I think all the theoretical stuff is interesting (if you are into fluid dynamics), but my only addition to this is that a few years ago after trying  for weeks and getting very depressed (no pun intended) about the state of the brake pedal in my 80" I took it along to a local garage,  explained the problem, told them everything I'd tried, they hooked it up to a mains powered pressure bleeding system via the slaves in turn, in the prescribed order, took about 20-30 mins, and I had a much much safer drive home :)  I think it went up to 4or 6 bar for ABS systems (but I could be wrong that's from memory!) but they only used a much lower pressure on the land rover.

I enquired at the time what the system they used cost, don't remember the brand, might have been sykes (it looked a bit like one of those big old battery chargers on wheels), they said it was about £500 (this was circa 10 years ago), but that it saved them so much time every time they had to sort out a brake issue on a customer car that required bleeding, and that it got them a firm pedal every time, such that, if some reason it didn't they were confident that there was a real issue (like a leak)

Obviously I'd already spent weeks checking everything, so the shoe adjustment etc was pretty much spot on.

Conversely my diesel 88" when I got that back on the road, I just filled the system, opened each slave in turn, waited for it to drain by gravity, and got a sold pedal first time. The 80" setup with the larger pipes etc. is just not very good!
 

Offline Ivor M

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Re: Tips on brake bleeding please
Reply #20 - Jan 10, 2022 - 09:47
Mike, you still get fluid flow with the vacuum approach, you could say that atmospheric pressure push it along. yep one end of the system is at ~14.5psi, the other depending how hard you pull on the syringe. yes pulling the syringe requires some force could easily be 7psi.

Offline TDC

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Re: Tips on brake bleeding please
Reply #21 - Jan 10, 2022 - 10:13
Shock tactics are sometimes needed.

Half opening those small bore bleed screws under pressure is like turning a tap on slowly. Air bubbles remain trapped or caught.

What's needed is to crank up the pressure and crack the bleed screw fully open (ie. big turn of spanner) as fast as possible to allow a sudden, fast flow of fluid to shift bubbles along and out. In addition, the tube into an empty jar needs to be large bore and short so it offers minimal flow resistance. Indeed, a small bore rubber tube and jar restriction may well be the cause of poor bleeding flow.

I realised this once after hours with a soft pedal I turned the compressor up (no advice please) and whacked open the bleed screw. The hose blew off with the jet of fluid! Instant result – firm brakes.
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Offline Trakgrip

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Re: Tips on brake bleeding please
Reply #22 - Jan 10, 2022 - 10:57
From the point of view of physics and engineering, I cannot see any theoretical difference at all between pressure bleeding and vacuum bleeding, since in both cases the pressure at the reservoir is higher than the pressure at the nipple. In the case of the former, the reservoir is raised above atmospheric pressure and the pressure at the nipple is at atmospheric to create a pressure differential, in the case of the latter the reservoir is at atmospheric and the pressure at the nipple is below atmospheric to create the pressure differential. If the relative pressures differ by the same amount, there should be no theoretical difference - at least none that I can see.

The only practical differences I can see are firstly that with vacuum bleeding a sealed braking system there is the possibility that air is drawn into the vacuum pump between the bleed nipple threads and the body of the wheel cylinder, and secondly with pressure bleeding any union at any point in the system can be "cracked" to allow bleeding.

I have found personally that pressure bleeding is more likely to achieve the desired result with a minimum of trouble if trying to fill and bleed a system which is entirely new and devoid of fluid. Provided that the system is largely filled with fluid, then I find either is effective.

Curiously also I have never (at least not yet) had trouble filling and bleeding a Series One from scratch. It is hard to see why some vehicles give so many problems, but it certainly seem to be the case. 

Offline TDC

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Re: Tips on brake bleeding please
Reply #23 - Jan 10, 2022 - 11:11

Curiously also I have never (at least not yet) had trouble filling and bleeding a Series One from scratch. It is hard to see why some vehicles give so many problems, but it certainly seem to be the case.

I agree. My experience above was on a Jaguar rear IRS inboard brakes - lots of nooks and crannies! The factories must have done it in seconds on the production line.

Offline antarmike

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Re: Tips on brake bleeding please
Reply #24 - Jan 10, 2022 - 11:22
From the point of view of physics and engineering, I cannot see any theoretical difference at all between pressure bleeding and vacuum bleeding, since in both cases the pressure at the reservoir is higher than the pressure at the nipple. In the case of the former, the reservoir is raised above atmospheric pressure and the pressure at the nipple is at atmospheric to create a pressure differential, in the case of the latter the reservoir is at atmospheric and the pressure at the nipple is below atmospheric to create the pressure differential. If the relative pressures differ by the same amount, there should be no theoretical difference - at least none that I can see.

The only practical differences I can see are firstly that with vacuum bleeding a sealed braking system there is the possibility that air is drawn into the vacuum pump between the bleed nipple threads and the body of the wheel cylinder, and secondly with pressure bleeding any union at any point in the system can be "cracked" to allow bleeding.

I have found personally that pressure bleeding is more likely to achieve the desired result with a minimum of trouble if trying to fill and bleed a system which is entirely new and devoid of fluid. Provided that the system is largely filled with fluid, then I find either is effective.

Curiously also I have never (at least not yet) had trouble filling and bleeding a Series One from scratch. It is hard to see why some vehicles give so many problems, but it certainly seem to be the case.
The difference is there is an absolute pressure difference of - 14.7psi when vacuum bleeding.  This is the amount below atmospheric pressure when there is a perfect (100%) vacuum). It is phycically impossible to create a lower pressure.

The is no theoretical limit to the maximum pressure thatr can be applied when pressure bleeding.  Easibleed recommend 20 psi max ( which is 33% higher than vacuum bleeding.)  There may be practical limitations to pressure bleeding but people have successfully applied 30 psi without anything exploding.

Therefore the pressure difference are not the same when Vacuum Bleeding and Pressure bleeding, so your statement "If the relative pressures differ by the same amount, there should be no theoretical difference" is incorrect. that is not the reality of pressure bleeding.

Pressure bleeding offers the chance of a significantly greater pressure difference.  Where "significant greater" I mean between 33% more at 20 psi and 100% more at 30 psi and a vacuum of -14.7 rounded to  -15psi.   

But based on a more realistic achieveable vacuum of -10psi the pressure achieved by pressure beeding is greater by 100% at 20 psi and 200% at 30psi.

 

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