Caliper piston over-retracting

[alan.barker]

have you checked the run out of both discs with a DTI and also endfloat on both hubs

[JK1]

I have experienced the exact same problem with excessively long pedal travel on the first touch of the brakes requiring a second dab on the pedal to establish a firm braking effort and higher pedal position. The brakes presented a serious risk in the event of a panic stop being required. The problem followed on from having replaced master cylinder, servo, fitting new armoured brake lines front and rear, rebuilding the calipers with new pistons and seals. Everything in the braking system had been replaced and brakes bled several times to no avail. The conventional wisdom would suggest that the system had air trapped somewhere. Eventually a friend and more able mechanic suggested the problem of piston travel and retraction. A simple experiment of fitting a spacer between the pad and the piston (when of course the car was static) dramatically reduced the pedal travel. The pads had not been replaced when the system was rebuilt and they had very little material left. A new set of pads delivered the perfect fix and the thickness of the pad material resulted in reduced piston travel. Not only has the pedal travel problem been eliminated resulting in a more immediate response but the 'feel' in the pedal has also been dramatically improved where the brakes now have a much more progressive quality. Rohan had earlier suggested in this thread that a new set of pads was a possible solution and in this case it produced the desired result. The answer had eluded me for a while and it is ironic that the fix could have been so simple.

JK1

[Hawksfield]

JK1

What amount of pad was remaining when you changed them, 30% or less. It may be that we need to keep plenty of meat on the pads, I would normally change them at 30% remaining material on a modern car.
I am interested in your resolution to the long pedal travel which I have always had on my +2 and have not changed my pads in years, better check them.

[rgh0]

The pads tend to wear into a wedge shape over time. I typically replace my pads when the thickest bit is around 4mm - The thinnest will be under 3mm typically at that point

This shape leads to a longer travel I believe as the wedge shape tends to kick the piston back further when pressure is released. In my Elan I always find when I put in a new set of pad the travel is significantly less and the pedal firmer as the pads are square between the disc and piston.

This may not be the source of the original problem but worth a check. if nothing else new pads place a different part of the piston over the seal which may alter the seal pull back if that's the problem

cheers
Rohan

[JK1]

In response to JH's question the pads on the front right corner were down to 15 - 20%. The pistons on this side partially locked on one day due to rust and had worn the pads disproportionally on this corner. The pads on the other side had 40% remaining. This event triggered the rebuild and I foolishly reused the old pads and replaced everything else in the system, however, the Girling servo was consuming brake fluid at an alarming rate and needed to be replaced anyway. After the rebuild (pistons,seals, master cylinder, servo and flexis), the brakes were positively horrible but the new pads solved the problem and I now have excellent brakes. I spoke to another Elan owner recently who highly recommends Greenstuff pads which he claimed had transformed his brakes.

JK1

[RotoFlexible]

Many thanks to all for the continuing replies and suggestions. To address some points that were brought up:

- As billwill pointed out and JK1 experienced, the symptoms are very similar to air in the system. When I first fitted the +2 brakes, I bled them repeatedly thinking that was the problem. Then a local guru, Carl Whitney, told me about retraction and the need to lubricate. That helped but did not eliminate the problem,, so I'm on my second try. Also, I can correlate the motion of the pedal to the take-up in the loose pads.

- I considered disc deflection. But, the effort needed to move the pedal through its excessive travel is constant and very light (fingertip). The pressure needed to deflect both discs would be heavier and would increase over the range of travel. Also, that would require one piston to be so stuck that the disc deflected before that piston would move, but both pistons move easily when air pressure is applied.

- Runout may be a factor when the car is moving (although there is no pulsing) but in this case, the car up on stands with the wheels off when I observe the problem.

- All seals are brand new from RD Enterprises. Their origin and quality is unknown, but they should all be the same.

- The pads have about 11/32" (about 8.7mm) left - about one-third worn, I would say - and are the same thickness throughout. I have the "damping shims" installed, which I think introduce a very slight wedging bias to the pads. The pads on each side of the caliper have the same amount of wear. The evenness of the wear (across the pad and between pads) leads me to believe that the mountings are square.

- Before reassembling, I will swap the inboard and outboard pistons in the caliper that is out of the car. If the problem persists and moves from the inside to the outside, I'll know it's a problem with the piston.

I expect to get back to work on the problem this weekend.

[robertverhey]

Yeah my money's still on a mechanical reason, rather than a hydraulic one. Perhaps a slight ridge on the inner edge of the disc facing that's preventing the pads hitting the disc squarely and is pushing them back when the pressure's off. Which is why I suggested swapping them over, perhaps the pads and discs got mix'n matched during the reassembly?

A few years back I had a dreadful time sorting the brakes on my lotus excel following a rebuild....I bled and bled and bled 'em, to no avail. Problem turned out to be the pads on one caliper not moving freely along the locating pins....

[Chancer]

Yeah my money's still on a mechanical reason, rather than a hydraulic one. Perhaps a slight ridge on the inner edge of the disc facing that's preventing the pads hitting the disc squarely and is pushing them back when the pressure's off. Which is why I suggested swapping them over, perhaps the pads and discs got mix'n matched during the reassembly?

That sounds like a very strong possibility.

[RotoFlexible]

I think my problem is solved. With moral support and bleeding assistance from Nick (elanner), I swapped the pistons side-to-side on the caliper that was out of the car, cleaning and re-lubricating. An air pressure test showed distinct improvement, so we refilled and bled the system. The left-side caliper (the one we worked on) now has no pad rattle whatsoever. On the right side, one pad has a slight rattle, much less than when I started this project. The excess pedal travel is essentially gone.

I can't say for sure why the problem occurred, and why it now appears to be fixed, but I'm pleased with the result. I probably won't drive the car for a couple of months but I expect the brake feel to be much improved.

Thanks to all who replied, and especially to Nick for his patience during three rounds of bleeding.

[RotoFlexible]

To follow up on this: after one last round of bleeding, and with the advent of proper spring weather, I was able to road test the Elan today. The brake pedal feel is much better. The inch or so of travel that I attributed to piston retraction is gone, and the brakes start biting almost immediately. Again, thanks to all.