Brakes seem to be holding on too tightly

[alaric]

That's brilliant. I have my old master cylinder, so could try to use that in the same way. You've made a really sturdy looking tool there. Thanks for posting the pic.

I went out to the garage but ended up gluing some carpet back to open up the heater duct in the passenger foot well. Seemed a much easier task. So I haven't started yet.

Sean.

[Baggy2]

FWIW I found that no amount of hydraulic pressure would shift a siezed piston - though I must admit I didnt have the ingenious rig posted by Foxie. The final solution was to split the caliper , Drill a 3/8th tapping hole in the bottom of the piston - tap it then screw in a bolt to force the piston out . As mentioned having split the caliper it is necessary to rplace the seal between the two halves . I'm pretty sure a company called PowerTrack will be able to help there - knowledgable and helpful folk
Hope this helps

[Andy8421]

Sean,

Unless you want to go down Baggy's route and drill the piston, and are planning on the pressure solution, don't split the caliper until you have both pistons out - and jam something in the gap the disc normally runs in to stop one piston coming out all the way leaving the other stuck piston in place. You can use a grease gun to do this if you can find a suitable grease nipple to fit the caliper - its a bit messy and wastes grease, but saves having to build Baggy's custom rig.

You can of course just leave the caliper on the car and just pump the brake pedal forcing the pistons out. The disc will stop the 'easy' piston coming out all the way and allow you to get the other piston out. Not a great idea to side load the disc, so I have done this before with the caliper mounting bolts removed, holding the caliper approximately in place so the disc is still in its gap while a helper pumps the brakes.

Good luck.

[2cams70]

Suggest you read my earlier post further up. Particularly about the quality of repro parts. Girling is best. Who knows what kind of tolerances are being used on the seal groove cut into repro. calipers. If it’s not correct or the repro seals aren’t the correct dimensions you will get sticking on new calipers

[alaric]

Hi all.

In my case the pistons are certainly not siezed, it's just that the piston does not pull back as far as it should. Braking power is great, and actually it stops when braking hard without pulling to the right or left - there's just an initial twitch to the right when I hit the brake pedal quickly. That's the pad being too close to the disc.

So I'll have a look and see what's going on. I see I can get seal kits and replacement pistons easily enough if I need them, and presumably on a much shorter lead time than sending the calipers away for refurbishment. It's getting a bit annoying that companies offering exchange calipers are actually offering refurb of my calipers, which means I'm without the calipers for as long as it take them to return them. I'm not doing that - I'll get the pistons out and take a look.

I'm not splitting the two halves of the caliper - although I see no reason not to - the manual is badly written isn't it. It says not to separate the two halves of the caliper, but what it actually means is don't put those halves together with a half from another caliper, as they are a matched pair. But I'm not taking it apart - I'll probably break the casting or something stupid like that haha.

Regards.

Sean.

[2cams70]

You guys just don't seem to realize how critical the profile of the seal groove in the caliper is. It's actually a special shape to pretension the seal so it retracts the piston slightly when the brakes are released. That's why if you have your original calipers and they are salvageable (usually they are) you are far better off keeping them than going with unknown repros.

https://www.sae.org/publications/techni ... 2-01-0927/

[Foxie]

After all these years, this is new to me !

Over time, I've fitted anti knock-back springs and residual pressure valves

However the link is looking for $33.00 to read the paper.

Would it be possible to summarise the main points ?

Thanks,

[RichardHawkins]

2 cams,

Thanks for posting the SAE paper about the shape of the seal groove. I have always suspected that the seal had two purposes, first to keep the fluid in and second to pull the piston back slightly. I would never have imagined that the shape of the seal groove was involved. I agree about reverse engineering not simply being a copy, as the revers engineer seldom has access to design data and often overlooks subtle detail.

Richard Hawkins

[alaric]

I knew

That's why I'm concerned that the piston is not retracting enough on that side. It's not seized - I have extremely powerful brakes on the car that stop it in a straight line. The slight twist that I sometimes get will be the piston not retracting enough and that will, I believe, be something to do with the seal or piston, which was replaced not long before I bought the car; the seal may be too tight - maybe the rubber is too hard or not put in correctly, or the piston may be slightly oversized. So this is probably a new problem with the car that the previous owner was certainly not aware of. I'll get it sorted. I may find one of the pads is more worn than the other, which would give me a clue.

Top priority this weekend is setting up my Carcoon in the garage though... get it nice and cosy.

Sean.

[nmauduit]

are you 100% sure this is the result of a brake issue ? that is, no play in steering, no steer bump coming from the geometry (mostly coming from steering rack positionning) ? if you can't feel a braking problem with the wheel hanging I would look into geometry, as the braking issue should not be dominant in the described behaviour...

[alaric]

Hi. Thanks for the suggestion but I'm pretty sure it's not suspension. The offside wheel hub won't spin on its own while the nearside does - it's much freer. And the offside disk gets warm, while near side stays cold. So whether there's a geometry issue or not there's definitely a problem with that caliper. I don't get any bump steer that I've noticed - the rack is carefully shimmed to avoid that too - at least that's my understanding of the blocks that are in there. I saw no movement in the joints when I checked.

Haha perhaps I just need to push the piston out far enough to get some wet and dry on there - joking, I'm joking

Regards.

Sean.

[nmauduit]

well I was under the impression that statically the car was more or less symetric: if the hub is getting hot from the brake constantly braking on one side you should take that side caliper apart soon, before embarking on any significant journey or more than a mile or two, as the possible outcome is boiling brake fluid (heat transmitted via the piston) then absolutely no brake at all...

[alaric]

Yes. Agreed. But to scale the problem most people would not have noticed this... it's just slightly rubbing. The car rolls freely etc etc. Caliper apart it is though... as it appears pro refurbishment is not so quick these days.

Sean.

[nmauduit]

well then, if it's only slightly rubbing that should not be what is making your car pull to one side under heavy braking...

the easiest to make sure is to check the offending caliper, then if all ok I would still double check the steering rack geometry.

good luck !

[Donels]

You say "the pistons are certainly not seized, it's just that the piston does not pull back as far as it should". I don't think that is correct. The pressure in the system will be the same at all calipers and, assuming the pistons are not seized, the pads will not apply braking force to any of the discs until all pads are in contact with the discs. (The major advantage of disc brakes is that they don't pull to one side or the other because of this.)

The distance the pad has to travel is irrelevant. If you have one caliper binding as you say I suspect that piston IS binding, therefore the seals are not capable of pulling the piston back. When you brake it is taking more force to move that piston or it's bending the disc because the piston is not moving.

I suspect the piston is fully seized and it's bending the disc, so the car pulls under gently braking, disc not bending, but not under hard braking when the disc is bending.

Dave