jhoppeolds

About 10 years ago I rebuilt the front disc brake calipers on my '70 Delta using aftermarket replacement phenolic pistons. At the time, no parts house seemed willing or able to get me rebuilt calipers, so I just decided to do them myself. The plating on the original steel pistons was peeling and the only replacement pistons available seemed to be the phenolic ones so that's what I got (Raybestos DP85010). The calipers were cleaned and lightly glassbeaded. I then went in the bores and lightly sanded them with 600 grit sandpaper just in case the glassbeading raised the surface a bit and tightened up the bore clearance. Unlikely, as the pressure I used was very low and there was no visible surface peening. The finished brake job worked flawlessly. I drove the car year round for about 5 years, then did an extensive resto but didn't touch the brakes except to change out all the brake fluid just as a mantainence thing. The car was now driven much less and only in the summer. Last year while cruising around the right front caliper started hanging up and dragging. Wasn't a swollen brake hose issue (they were replaced previously and opening the bleeder while the caliper was dragging didn't release it), so I pulled the caliper and disassembled it, expecting it to be full of crud after 10 years, but guess what, it looked perfect. The caliper slides seemed a bit stiff so I cleaned them up and put it all back together and the brakes worked good again. At least until last week when the same thing happened. Same scenario, haven't taken it apart yet, but I'm wondering if there isn't a bore/piston clearance problem after everything heats up and why did it take 10 years for the problem to manifest itself. The second time I reassembled the piston into the bore I first put it in without the seals and it did slide in and out smoothly but seemed like the clearance was mightly small (had to wiggle it a bunch to get it started down the bore). Seemed really tight once the inner seal was installed but can't recall if it was that way the first time around. Is there a steel piston available or has anyone had a similar experience?

Olds64

That is an interesting problem you are having. Perhaps your right front caliper is hanging up because you have a faulty master cylinder. If you have a dual chamber master cylinder then the right front brake caliper and left rear slave cylinder are activated by one chamber of your master cylinder. If it is a faulty MC you wouldn't necessarily see the left rear slave cylinder hang up because the drum brakes require a greater amount of movement to activate than disc brakes.

FWIW, I don't believe there are any parts available for the pre 1971 B and C body disc brake setups.

jhoppeolds

Interesting theory, but both my front calipers are fed by the front (large) resevoir on the master and both rear wheel cylinders by the smaller rear resevoir. I suspect both calipers would be dragging if there was a master or proportioning valve issue. I did once encounter a situation where a drum brake dual resevoir master was installed on a disc brake car and caused both front calipers to drag. I think there is a residual pressure check valve in the very end of the fluid outlet port on the master on drum brake cars to help overcome the brake shoe return springs upon initial application of the brakes.

Rickman48

Think I'd pull it apart again, and look for 'galling' in the cylinder, or a crack in the piston! Something's hanging up, obviously!
Although 10 years is pretty good longevity for a rebuild - i'd still want to know, myself!

Olds64

Oops, I guess my logic was messed up.

Good luck solving your problem.