Brake Shoe Position on Brake Cylinder

Little Dog

A2OC Donor
European-Union
I noticed the rear most brake shoe appeared to be about to slip off the brake cylinder while preparing my car for its MOT test:



The front shoe is positioned centrally on the piston:



The brake assembly looks like this:



I have checked these two threads:

http://www.a2oc.net/forum/showthrea...el-cylinder-piston/page3&highlight=rear+brake

http://www.a2oc.net/forum/showthrea...r-drivers-side-brake-drum&p=203535#post203535

The only thing I have noticed is that there is a lip on one edge of the cylinder in front of the shoe. Should this be behind the shoe? Any advice appreciated.
 
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I noticed the rear most brake shoe appeared to be about to slip off the brake cylinder while preparing my car for its MOT test:



The front shoe is positioned centrally on the piston:



The brake assembly looks like this:



I have checked these two threads:

http://www.a2oc.net/forum/showthrea...el-cylinder-piston/page3&highlight=rear+brake

http://www.a2oc.net/forum/showthrea...r-drivers-side-brake-drum&p=203535#post203535

The only thing I have noticed is that there is a lip on one edge of the cylinder in front of the shoe. Should this be behind the shoe? Any advice appreciated.
the lip on the cylinder piston should be in front of the shoe , there are two different cylnders /shoe sizes depending on model , if all parts are correct sizes (fairly obvious visually ) then common practice would be to unwind handbrake fully at rear of h/brake lever to fully release mechanism then if drum condition is good no lips, would be to then adjust shoes out so as the drum will spin freely when fitted but any more adjustment would cause a bind , once both sides have correct adjustment adjust h/brake again to a few notches , if all is correct then you should find the shoes will sit centrally on pistons ,
 
Thanks Mark; that is exactly the procedure I apply on a classic mini. Never thought it was necessary on the VAG cars that I have owned as the rear drums and callipers have all been self-adjusting.

To remove the drum I simply spring the adjusting wedge and the drum comes off easily.



It goes back on easily and with two presses of the brake pedal the shoes are correctly adjusted. Would I be missing a trick somewhere? Also is the position of that shoe an issue?
 
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Thanks Mark; that is exactly the procedure I apply on a classic mini. Never thought it was necessary on the VAG cars that I have owned as the rear drums and callipers have all been self-adjusting.

To remove the drum I simply spring the adjusting wedge and the drum comes off easily.



It goes back on easily and with two presses of the brake pedal the shoes are correctly adjusted. Would I be missing a trick somewhere? Also is the position of that shoe an issue?

It is also worth checking that the little "buttons" are in place. The edges of the shoe sit against these and they look like little metal discs with a very short centre pin.

These can fall out when working on the shoes and that would make the shoe sit closer to the backplate than it should be.

Steve B
 
Thanks Steve, I started investigating because the rear brakes suddenly started making a scraping noise. They were full of dust so assumed this was the source of the noise.

Not so; the scraping noise is still there and I now suspect it is the out of position shoe being applied against the small rough ring of drum on the inside (where a lip eventually forms). I’m now thinking something broke in use, also I did retrieve something looking like a white nylon washer when I cleaned the back plates. Any ideas? Could that be part of the pin?
 
Thanks Steve, I started investigating because the rear brakes suddenly started making a scraping noise. They were full of dust so assumed this was the source of the noise.

Not so; the scraping noise is still there and I now suspect it is the out of position shoe being applied against the small rough ring of drum on the inside (where a lip eventually forms). I’m now thinking something broke in use, also I did retrieve something looking like a white nylon washer when I cleaned the back plates. Any ideas? Could that be part of the pin?

most certainly the white ptfe 'washer' is the remains of one of the 4 'pads' that the shoes slide against on the back plate
 
Well the new pads failed to correct matters. They are approximately 1mm thick so moved the shoe forwards by 1mm and so it still sits on the edge of the piston.

Any suggestions? Perhaps a machining error on the cylinder????
 
Got to the bottom of this and yes it is a cylinder geometry problem. Here is the rear most shoe after the pads were replaced on the back plate:



Notice the white line between the aluminium cylinder and steel back plate? That is corrosion that has built up over time and forced the rear of the cylinder away from the back plate creating a risk that the shoe will slip off the piston and cause brake failure. More pictures of the cylinder:





There is one securing bolt on the leading edge and the corrosion jacks the trailing edge away from the back plate.

Interestingly I suspect the issue has been recognised and new cylinders are iron and come with two threaded holes for securing:



If you have got them (aluminium cylinders) check them.

Edit I: two threaded holes results in one part number for LH and RH cylinders. Could be used though for a second screw if there is any build-up of salt behind the cylinder. I suspect though, in addition to salt, the white crust I removed was the result of dissimilar metals being in contact causing the aluminium cylinder to corrode.

Edit II: No that second hole has a purpose; N/S back plates secure the cylinder forward of the axel and O/S back plates secure the cylinder behind the axel. There was one cylinder that fitted both sides.
 
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Worked today on the O/S, same problem developing but this time on the forward most shoe:



Same problem, corrosion behind the cylinder forcing it away from the back plate:







New bits fix the problem nicely:



Scraping noises gone :)
 
Glad you got this fixed.
I hope that my thread helped. http://www.a2oc.net/forum/showthread.php?27472-brake-shoes-slipping-off-wheel-cylinder-piston
I replaced the one back plate and both cylinders and shoes (Brembos) and have had no problems with scraping sounds since then.
In fact the rear brakes are much better than ever, before this I was always having issues and never got to bottom of it until I replaced the one back plate and both cylinders and shoes. The handbrake has just been adjusted for the first time in two years.
I just had an advisory on the front brakes and intend to get some quality parts to replace them - you know it makes sense...
 
Hi Au2ro, your thread was my starting point and it looks like you had quite a job. It was reassuring you had fixed the problem.

I also wanted the root cause and that was the corrosion build up behind one end of the alloy cylinders forcing them out of position. I believe this also increased the wear on the back plate slipper pads. The alloy cylinders I replaced were fitted by myself about 10 years ago, were VW/Audi, and replaced iron cylinders that had been fitted in the factory. I don’t recall any issues with the original iron cylinders being forced out of position (one was leaking and shoes were binding) so I believe this problem is now history but I will be checking at service time.

The brake kit I fitted this time is a Pagid pre-assembled kit with cylinders and grease etc. I believe original equipment? and at £43 a very cheap fix.
 
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