Help - Hot rear drums

[markevo30]

This one's doing my head in a bit.

I've recently replaced the front shoes, cylinders and flexi hoses and I've also had a remote servo fitted so performance is good. Out the back I've had new cylinders and shoes fitted.

So I thought I'd just got my brakes in tip top condition but on the way to the EB Campout I noticed that the rear drums were getting really hot. Too hot to touch.

At first I thought it was the handbrake adjustment so I slackened that right off but the drums were still getting very hot. Then I backed off the star adjusters all the way and the problem was still there. After a bit of reading it looked like it might be the flexible hoses so I've just changed them both. After a load of bleeding and being very careful with the adjustment of both the star adjusters and the handbrake I took the van out for a test run and the drums are still getting hot :evil:

I'm at a bit of loss as to what to look at next as pretty much everything is new.

One thing I did notice was when I had the drums off I got my son to press the brake pedal so the pistons pushed out the shoes. When he took his foot off I would have expected the shoes to return, but they didn't, I had to push them back. Could it be something as simple as the return springs not being strong enough? Should the spring pull the shoes back once the brake pedal has been released?

Any suggestions would be gratefully received.

Thanks

Mark

 

[sparkywig]

Definitely something binding there, sounds like the servo is keeping the brakes on.
Can you disconnect it and give that a try, just need to press the pedal a bit harder for a while.

 

[Trikky2]

Yes the spring should pull the shoes back.
Repeat the test and while the shoes are stuck, try slackening off the bleed valve. If the shoes retract then the problem is hydraulic. If they stay stuck then it's mechanical (either too much friction or Spring too weak).

Note : only press the pedal a little just so you can see the shoes move outward. If you press the pedal right down then the cylinder will over extend and the piston will jam in the bore giving the impression it's sticking).

 

[markevo30]

Definitely something binding there, sounds like the servo is keeping the brakes on.
Can you disconnect it and give that a try, just need to press the pedal a bit harder for a while.

It would take a fair bit of plumbing to disconnect the servo. It's a bit of a last resort but I may have to bite the bullet and do it in the end.

 

[markevo30]

Yes the spring should pull the shoes back.
Repeat the test and while the shoes are stuck, try slackening off the bleed valve. If the shoes retract then the problem is hydraulic. If they stay stuck then it's mechanical (either too much friction or Spring too weak).

Note : only press the pedal a little just so you can see the shoes move outward. If you press the pedal right down then the cylinder will over extend and the piston will jam in the bore giving the impression it's sticking).

Thanks, that sounds like a plan. I like the idea of narrowing it down in one easy move. I'll give it a go and report back.

 

[rlepecha]

Definitely something binding there, sounds like the servo is keeping the brakes on.
Can you disconnect it and give that a try, just need to press the pedal a bit harder for a while.

It would take a fair bit of plumbing to disconnect the servo. It's a bit of a last resort but I may have to bite the bullet and do it in the end.

I think what sparky was trying to say is just disconnect the vacuum from the Servo, and then it shouldn't be doing anything.

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[Moseley]

Most likely culprit as lots of parts are new, is the pistons themselves being gunked up or worn with age. Our last van had exactly the same and it was a head scratcher. Turned out that although the pistons were clean, they had worn into a slightly out-of-round shape and therefore were binding slightly and wouldn't retract. New pistons purchased, and all was fine after.

 

[markevo30]

Definitely something binding there, sounds like the servo is keeping the brakes on.
Can you disconnect it and give that a try, just need to press the pedal a bit harder for a while.

It would take a fair bit of plumbing to disconnect the servo. It's a bit of a last resort but I may have to bite the bullet and do it in the end.

I think what sparky was trying to say is just disconnect the vacuum from the Servo, and then it shouldn't be doing anything.

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Yeah, thanks for that, i was being a bit thick

 

[markevo30]

Most likely culprit as lots of parts are new, is the pistons themselves being gunked up or worn with age. Our last van had exactly the same and it was a head scratcher. Turned out that although the pistons were clean, they had worn into a slightly out-of-round shape and therefore were binding slightly and wouldn't retract. New pistons purchased, and all was fine after.

Are you referring to the pistons in the rear cylinders? If so it cant be that as they're brand new. The search continues.....

 

[Moseley]

Most likely culprit as lots of parts are new, is the pistons themselves being gunked up or worn with age. Our last van had exactly the same and it was a head scratcher. Turned out that although the pistons were clean, they had worn into a slightly out-of-round shape and therefore were binding slightly and wouldn't retract. New pistons purchased, and all was fine after.

Are you referring to the pistons in the rear cylinders? If so it cant be that as they're brand new. The search continues.....

Apologies, clearly skim reading.... As trikky said, with the shoe stuck out, try slackening off the bleed nipple. This will eliminate a problem with the fluid supply to the piston if it still doesn't retract.

 

[markevo30]

Most likely culprit as lots of parts are new, is the pistons themselves being gunked up or worn with age. Our last van had exactly the same and it was a head scratcher. Turned out that although the pistons were clean, they had worn into a slightly out-of-round shape and therefore were binding slightly and wouldn't retract. New pistons purchased, and all was fine after.

Are you referring to the pistons in the rear cylinders? If so it cant be that as they're brand new. The search continues.....

Apologies, clearly skim reading.... As trikky said, with the shoe stuck out, try slackening off the bleed nipple. This will eliminate a problem with the fluid supply to the piston if it still doesn't retract.

No need to apologise, all help gratefully received.

 

[Hilly_70]

I had the same problem a few years ago .. changed loads of stuff, cylinders, springs etc...

Still did it ..

What it turned out to be was the push rod into the master cylinder didn't have the required gap between the push rod and inner cylinder (think it should have a 2mm gap)

What was happening was when it got warm, it wasn't returning the cylinder quite far back enough to allow the fluid to return and continuously building pressure .. for some reason it only affected the rears..

Slacked off push rod (30 second job!) and all was happy again ..

Not saying its that .. just my experiences .. but worth a check

Hilly

 

[Max Power]

I had exactly the same problem,only done it when warmed up after a couple of miles.
Turned out to be the pushrod wrongly adjusted as above after I changed the master cylinder.

 

[markevo30]

Thanks for pushrod tips guys. I've checked it when cold and it seems ok but I'll have another look when warm with the engine running to see how it is with the servo working.


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[markevo30]

I've finally had chance to have another look at my brakes. I checked the pushrod clearance and found I had at least a couple of mm of play so I ruled that out. Next I followed Tricky 2's advice to see whether the problem was mechanical (weak springs or sticking shoes) or hydraulic.

With the drums off I pressed the pedal down just enough for the slave cylinder pistons to push the shoes out. When the pedal was released they stayed out. I opened up the bleed nipple and the shoes slowly moved back to their original position. So there's something wrong with the hydraulics then.

I repeated the test but moving the point where I released pressure from the system. First I looked at the output from my remote servo and then the input with the same result. The last point I tried was the output from the master cylinder, again when I cracked the brake pipe union the shoes returned to their normal position.

Surely this points towards the master cylinder being goosed? It can't be anything else right?

 

[Bluesnailman]

i follow your logic, yip. one point though. the mc does (on mine at least) have a one way valve on the output. Now I know its one way, but i think the sprung portion of it allows just enough slackening of pressure for the shoes to return slightly when the pedal comes off. perhaps something odd is happening here (rather than the main body of the mc).

as usual, others will have an opinion...

 

[Moseley]

Seems like you've done some thorough checks to rule things out - as above, I'd at least try stripping and cleaning out the master cylinder and then re-assemble. It hasn't cost you anything then, and you may find that it is full of shite and can be rejuvenated, or is corroded internally and needs replacing for a new item.

 

[Moseley]

And thinking about it further - can the secondary port on the MC actually retract independently of the primary one? Although they will both be pushed together when the brake pedal is pressed, I thought the secondary piston could in theory remain in the 'brakes on' position, even when the pedal is released. Obviously this would be caused by some extra friction in the back of the MC if there was corrosion or damaged seals maybe.

 

[markevo30]

And thinking about it further - can the secondary port on the MC actually retract independently of the primary one? Although they will both be pushed together when the brake pedal is pressed, I thought the secondary piston could in theory remain in the 'brakes on' position, even when the pedal is released. Obviously this would be caused by some extra friction in the back of the MC if there was corrosion or damaged seals maybe.

I've just had a look at an exploded picture of a master cylinder and it looks like there's a separate piston for the front and rear brakes. The rear brake piston is at the push rod end so it is more susceptible to corrosion or contamination so before i shell out for a new M/C i'll have a look at the old one to see if there's an easy fix.

 

[markevo30]

I've removed and stripped the master cylinder and it all looks good apart from the amount of gunge that was in the reservoir which i've now cleaned out. There's no corrosion and all four ports are clear. I thought that there may have been some blockage due to gunge so i put it all back together and tried it. Same problem, that is the brake shoes are still not returning after the brake pedal is depressed and then released.

I took the m/c off again this morning to have another look to see what could be going on. When i had a proper look at where the brake pipes go onto the m/c there are one way valves. Surely this is what's causing the problem? Are these non return valves supposed to be there? If they are then how is the pressure built up when braking going to reduce when the pedal is released and allow the shoes to return?

Gunge in the reservoir



The m/c seems to be in good nick



Are these non return valves the cause of the problem?