R66

I have been fighting the rear calipers on R66 sucking air. Thought I had it fixed by getting the wheel bearing and disc run out within tolerance.
On our trip to the FunFest on Thursday of last week, the brakes were fine until I had to make a hard stop due to a garbage truck. The car stopped straight and very well. I was happy.
About 10 miles down the road, the right rear caliper began dragging.
Stopped, backed up and turned around to find a repair shop. After turning around, the caliper released and everything was good except a pull to the left when the brakes were first applied (air again).
Got home and put new Lone Star O-ring calipers on the rear. Checked run out and every thing again. On our trip to the FunFest on Saturday, everything was good. Just hope it stays that way.

I decided to find out why the old calipers were malfunctioning and tore down the right rear caliper to find the SS bores were from 1.476" to 1.479" I.D. - GM spec is 1.375" I.D.
Measured the pistons and found they varied from 1.368" to 1.370" O.D.
The GM allowable piston to bore clearance is .0035" to .009". I found over .100" clearance on every bore (over 10 times the max.).
Tbarb and Dub correct me if I am wrong, I guess the excess clearance allows the pistons to rock in the bores and thus leak, lockup, and suck air. I thank you all and others for your past help in trying to resolve this.

I love cheese, but hate whine, so please forgive me. I just cannot understand how rebuilders (unknown as the calipers were on the car when we bought it) can put out JUNK that puts the lives of those that use their product in peril.

Be aware this JUNK is out there.

Ron

emdoller

Sad to see that. Where did you get them?

Ed

DansYellow66

That's pretty sloppy. I wonder if they might have been rebuilt before and the SS bores honed out excessively? Although you wouldn't think SS bores would need honing during a seal replacement.

tbarb

The problem you describe sure sounds like a rubber brake hose that's not allowing the brake caliper to pass fluid back and release. Also, have you changed anything around the master cylinder, because a incorrect part could keep the piston blocking a port allowing fluid/pressure back.

I don't have the brake caliper specs but a .005 -.009 + - clearance between pistons and stainless bore sounds tight to me, double check that. Are you sure the figure is not .035-.090, lip seals take up quite a bit of space and I can't imagine installing them with that tight clearance. My feeling is that the pistons will not rock in their bores with the lip seals. You did good correcting the rotor run out, I did that on my 67 earlier this year.

wmf62

the ones I had on my 62 had SS sleeves and also had SS pistons, I believe they were some of the very first ones made by SS Brake Corp. after many years of OEM leakers I replaced them with the SS; I don't remember ever having a problem. I finally replaced them with some oversize single piston calipers when I installed the IFS; I don't have a clue as to how well those will hold up....

I never had cause to measure, so I can't comment on that aspect

Bill