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Discussion Starter #1 (Edited)
Regular readers may recall that I have had an ongoing issue with erratic excessive free travel at the brake pedal. And, that I've had some success in reducing this by increasing the friction in the rear caliper pins (to minimise the rear caliper body 'flopping' around on the pins).

This was achieved by wrapping the 'insert pins' (the ones with a rubber insert near the inner end of the pins, one per caliper) with shim foil. The shimming material was placed between the pin and the - probably worn - rubber insert, expanding the insert into tighter contact with the pin bore in the caliper mount. Also, the plain steel 'insertless' pins (one per caliper) were machined with four grooves in the pin shafts into which small O rings were fitted. The O rings create friction between the pin bores and the pins. With the increased friction at both pins in each caliper, the pins still slide, but not entirely freely, some force is needed to move the pins in / out (and the caliper body no longer 'flops around' on the pins).

All this reduced excessive 'free' in / out movement of caliper body on the pins, resulting in decreased pad 'knock back' and a significant decrease in pedal free motion (i.e. pedal motion before the pads made firm contact with the discs). Despite this, I could still manually move the caliper bodies in / out if I pushed and pulled hard enough on the caliper, by maybe half a millimetre or so.

Replacing the front caliper rubber pin bushes also made a significant difference to the pedal free motion, even though the old bushes didn't seem all that bad. None of the things I did to the front or rear pins caused any problems, i.e. no excessive pad drag etc.

But, there was still an issue, the problem was lessened, but not eliminated. The pedal would still be erratic, sometimes with very little free motion, sometimes with significantly more free motion. If the pedal was 'firm', then it was likely to be firm for maybe a day or so, then slowly become 'looser'. When pedal motion became 'loose' it might be 'loose' for a day or maybe a few days, then become 'firm' again, sometimes quite suddenly, and inexplicably.

WTF?

Suspicion foccussed briefly on the MC, but the pedal never moved toward the floor with prolonged foot pressure applied, which would normally be what indicates a problem with the MC (pressure leaking from the cylinder back into the reservoir). This suspicion was not forgotten, but placed on the back burner as somewhat unlikely.

So, for want of a well anylised diagnosis, I had my parts donor car with calipers that were approximately 60,000km newer than the calipers on my driver car (assuming no caliper had ever been replaced or rebuilt on either car). I pulled a rear caliper off the donor car and dismantled it (only as far as removing the piston). Before dismantling the piston rotated as it should but with some resistance. I cleaned it all up and inspected the seal and the piston, finding the seal to be visually perfect (well 99%) but with some black deposits adhered to the piston at the point around which the seal 'grips' the piston. I cleaned this off with a scotchbrite pad (finding the piston surface to be perfect underneath the deposit) and re-assembled the caliper using a smear of rubber grease on the seal and the piston OD. The piston now rotated more easily than before (rotation being related to how easily the piston moves in or out in the bore).

I then had a close look at the caliper which was being replaced, and found the piston to be significantly harder to rotate, but not significantly or actually seized. The seal looked fine, but black piston deposits were heavier than on the other caliper piston.

With the 'new' caliper back on the car and bled, the pedal felt better when driving the car. So, I did the same with the other rear caliper (swapped it for the newer and cleaned caliper from the donor car), better again. So I decided to do the same with the front calipers. The pistons in the front calipers were not 'seized', I could manually move them back into the bores with some effort. On disassembly I found similar black deposits on the pistons, but the seals themselves were visually perfect. I cleaned the pistons (finding the surfaces to be in perfect condition) and re-assembled the calipers with rubber grease. The pistons were now significantly easier to manually move in the bores.

A further improvement. On the road, the brake pedal is now consistently fantastic with almost no free motion. The pedal is nearly always 'right there' when my foot pushes the pedal pad. On occasion, there is a minor increase in pedal 'play', but it instantly disappears with a quick light 'tap' on the pedal. This was not previously the case, when a 'tap' might or might not cause the free play to disappear. Braking feels much better, more confidence inspiring, and 'heel / toe' is much easier due to the much improved consistency in free play and 'working' pedal height.

So, if none of the pisons were actually seized or particularly hard to move manually (i.e partially siezed), why has cleaning the pistons and lubricating the seals / pistons had this excellent effect?

My working theory is that the deposits on the pistons may create drag at the seals. This drag isn't enough to stop the caliper working, but may be causing the seal to 'stick' to the piston, enough so that when the piston moves outward it 'drags' the seal outward with it to an extent that elastically deforms that part of the seal which protrudes past the edge of the seal groove (in the bore). Upon release of pressure, the seal then retracts the piston to an extent beyond that which it is designed to do, causing excessive clearance between the piston, pads and disc. The piston still 'slips' on the seal when brake pad wear opens up enough the clearance between the piston / pads /disc, so the caliper still 'self adjusts', but not adequately to maintain a consistent minimal piston / pad / disc clearance, or consistent minimal free play at the pedal.

This would explain why the free play at the pedal was inconsistent. 'Self adjustment' wasn't smooth and progressive as the pads slowly wear down, but abrupt and intermittent, so sometimes there was minimal clearance between the piston / pads / disc, and sometinmes the clearance was excessive.

This won't neccesarily occur to the same degree at each caliper. Sometimes one caliper may have an excessive clearance and the others not. Sometimes this may mean all the calipers have excessive clearance, then one piston may 'slip' on its' seal to have a 'correct' clearance. At any given time all the calipers may have correct clearance, at others none may, or any combination. Maybe only one caliper is affected, maybe all four are. This would also help explain the problem being erratic.

It's only a theory...

So, if you have pedal free play issues then this might be a sign that the calipers need a good internal cleaning, particularly the surfaces of the pistons to ensure that they slide easily and smoothly on their seals.

All I can say with authority is that my - repeatedly bled - brakes used to be pretty bad with respect to having an erratic pedal height with excessive free motion. This was much improved by 'tightening up' the rear caliper pins and replacing the front rubber pin bushes, but after having fitted calipers with cleaned pistons and seals are now hugely improved (I am assuming that if the original calipers had been internally cleaned then the result would probably have been similar).

Regards,
John.
 
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