Binding back caliper - I think

[danrc]

I have recently upgraded my entire brake set up with a VTI servo, prelude front discs and pads plus the VTI rear discs and pads. I recently had to change the pads on the back as they were very low - so much so that they have left a small groove. The back left was at the minimum but the back right wasn't that low at all! I figured the caliper may have seized so i took them both apart, cleaned them up and refitted them with new pads.

Took the car for a test drive and noticed a very hot burny smell, looked at the back left wheel (The one which had gotten really low) and smoke was coming from the pads. :S Not good.

I took the car home, stripped the calipers down again and tried to grease the sliders (They were in decent nick anyway but i cleaned them and regreased them anyway), made sure all the excess brake dust was off the retainers and applied lots of copper grease everywhere on the moving bits. Same thing happens - get up to a decent speed and it starts to get over hot and smokes! Grr.

So what next? I am going to take the caliper off (Again!!) and get someone to apply brake pressure so the piston comes out, i will then clean the piston up and make sure its working ok, then push it back in with a G clamp and see how it goes. What grease should i use on the piston? I have silicon grease which is safe for rubber and copper grease.

Any ideas or tips would be much appreciated as i'm getting fed up rapidly.

[dilbert]

I don't know if you have handbrake shoes, is it possible that the pads and disks are fine, but that the handbrake shoes are rubbing?

[danrc]

Na mate its just just the brake pads, i know what you mean though. I tried to loosen off the handbrake and i have now adjusted it properly but still the same problem. I think its a sticky piston but i could be wrong.

[dilbert]

I'd say then that the most likely scenario is that the piston/cylinder interface has a bit of muck in it. Calipers can be funny beasts, mainly because the sliding fit is so long. The clearances are reasonably large, and the rubber seal does most of the sealing work.

This all adds up to a situation where a small piece of stiff muck, (rust possibly - that's usually my problem because I always have old cars, but I don't know how new your calipers are) gets into that gap, and can't get free.

The thing is that you have massive force to apply the brakes, and none to release them. If the piston and cylinder are binding, it's easy to overcome the wedging action of the muck with the brake pedal, but once it's released the pads are just wedged against the disk.

I think I'd take the caliper off, and pull the pistons out. When you put them back use new seals, and have both piston and cylinder spotlessly clean. If there's any rust at all, get a strip of fine wet or dry paper, wrap it around the piston and rotate the piston in the paper.

Same is true for the cylinder, although this is more fiddly. Whatever you do, dont try to use the piston to clean out the cylinder with the wet or dry paper. You're not looking to change the size of the parts, just remove any light corrosion.

Mine have been heavily corroded in the past, and virtually the only thing you can do is fit anew.

Does the caliper float, or are there pistons either side of the disk? If the caliper is fixed, and you have a piston either side of the disk, is it only one side that's wearing excessively or both?

Edited by dilbert on Sunday 8th April 21:10

[danrc]

dilbert said:

I'd say then that the most likely scenario is that the piston/cylinder interface has a bit of muck in it. Calipers can be funny beasts, mainly because the sliding fit is so long. The clearances are reasonably large, and the rubber seal does most of the sealing work.

This all adds up to a situation where a small piece of stiff muck, (rust possibly - that's usually my problem because I always have old cars, but I don't know how new your calipers are) gets into that gap, and can't get free.

The thing is that you have massive force to apply the brakes, and none to release them. If the piston and cylinder are binding, it's easy to overcome the wedging action of the muck with the brake pedal, but once it's released the pads are just wedged against the disk.

I think I'd take the caliper off, and pull the pistons out. Does the caliper float, or are there pistons either side of the disk? If the caliper is fixed, and you have a piston either side of the disk, is it only one side that's wearing excessively or both?

Thanks for the reply!

The caliper is a single piston which acts on one side and uses 2 sliding pins. I think your right and because the piston has been out a fair distance for a while (The pads have been low for a while) and rust and muck has accumulated. Will taking the piston out not release the brake fluid and so need re-bleeding? I will try and get the piston out as far as is safe and get the old wire brush out and use plenty of silicone grease.

Other than this problem my brakes are amazing! I have never had brakes as good, it is just really annoying!!

Thanks again mate, the help is much appreciated.

[dilbert]

danrc said:

dilbert said:

I'd say then that the most likely scenario is that the piston/cylinder interface has a bit of muck in it. Calipers can be funny beasts, mainly because the sliding fit is so long. The clearances are reasonably large, and the rubber seal does most of the sealing work.

This all adds up to a situation where a small piece of stiff muck, (rust possibly - that's usually my problem because I always have old cars, but I don't know how new your calipers are) gets into that gap, and can't get free.

The thing is that you have massive force to apply the brakes, and none to release them. If the piston and cylinder are binding, it's easy to overcome the wedging action of the muck with the brake pedal, but once it's released the pads are just wedged against the disk.

I think I'd take the caliper off, and pull the pistons out. Does the caliper float, or are there pistons either side of the disk? If the caliper is fixed, and you have a piston either side of the disk, is it only one side that's wearing excessively or both?

Thanks for the reply!

The caliper is a single piston which acts on one side and uses 2 sliding pins. I think your right and because the piston has been out a fair distance for a while (The pads have been low for a while) and rust and muck has accumulated. Will taking the piston out not release the brake fluid and so need re-bleeding? I will try and get the piston out as far as is safe and get the old wire brush out and use plenty of silicone grease.

Other than this problem my brakes are amazing! I have never had brakes as good, it is just really annoying!!

Thanks again mate, the help is much appreciated.

I posted a bit extra above, about cleaning the pistons and cylinders. Snce the caliper floats, you want to try and make sure that the slide pins are clean. It's not as critical as the pistons because you normally have a rubber bush that the pin slides in. That would normally have a bit of "give" in it.

I think you might do more damage than harm, unless you remove the cylinder completely. The last thing you want to do is push any more muck into the caliper.

You would have to bleed the brakes if you pull the piston out, but it's not that much hassle to get the job done right. If I were you, I'd take the caliper off completely if you're going to take the pistons out. It'll be much easier to work on, and you're going to have to bleed the brakes anyhow.

[dilbert]

Another thought is that if the muck is behind the seal, you'll never get rid of it, unless you take the piston out. Just as a way of looking at it, in normal operation more of the piston is behind the seal than in front!

Hope you get it sorted.