[bobzdar]

Originally Posted by Baron Von Zeppelin

Are they old used calipers or new ? Or reconditioned ?

I recently went through a sort of similar episode with the rear disc calipers on my dads 04 Tahoe after doing front/rear brake pads.
Rears would get Hot on test drive. Really Hot

Turned out to be the calipers had bad spots or maybe gunked up deeper up inside the bores and would not retract properly in their new positions with new thick pads.
Had been rarely driven in the past 15 years before he bought it about a year ago with only 65k miles on it. Rear pads were originals and inboard pads were really thin.

Put the old pads back on and it did fine.
He said leave it - I said NO . lol

He didn't want new calipers , so I made a compromise that worked out.
... a whole 'nother story ...

Everything is new except the metal lines - I converted from drums to discs so it has new calipers, hoses, mc, booster and prop valve. I would suspect the metal line if only one wheel were sticking, but both makes me think MC or prop valve. I called up firebird central (which is where I got the mc, booster and prop valve) to see if maybe they sent me the wrong (drum/drum) prop valve and they don't even carry a drum/drum setup, so doubtful. It's really odd, and if everything wasn't new I'd start from the wheels and work my way back but I feel like it's up stream from that...

[Baron Von Zeppelin]

First I would work the pedal a little bit , then break loose a bleeder on a caliper and see if fluid shoots out.
It should just dribble at the most.
See if the lines are holding pressurized fluid that way.

If you pass that test -

I'd take a caliper loose and artificially simulate the thickness needed to fill the gap from piston to the housing.
Leave about 1/4" gap for movement.
Have someone press the pedal and release while you watch the piston move and retract.
Then do again with 1/2" gap and see if any improvement in retracting.

New Chinese calipers could be flawed or defective or junk.
fwiw

Problems like this usually involve the caliper or the caliper hose.
Your new hoses should be okay

[bobzdar]

I did some more testing and the brakes seem to hold for a second or two before releasing. I didn't take it for a drive but they are not dragging or holding at all when I test the brakes parked. I was able to notice them taking a second or two to release by driving on a sloped area of my driveway, if I let off the brakes, the car wouldn't start rolling for a second or so and then you could feel the brakes release. They may just need to be bedded in, or it may be something funky in the m/c not retracting properly. Could low vacuum on the booster cause that potentially?

[Baron Von Zeppelin]

Booster problems usually give less effective brakes.

I had to block sand the pads with 40 grit to skinny them up , and then finish with 80 grit , to get to a point where my Dads calipers would retract properly.

Had to do that trial and error process twice to get satisfaction.
But saw improvement the first time - and continued the process.
And was using a slide caliper (measuring tool - not car part) to measure before and after thicknesses.
Was taking about 1mm off each time.

For full disclosure I actually was re-using his old outer pads that still had some meat left.
Maybe half worn
So was only sanding down 2 of the new pads to use on the inner side against caliper where most of the wear occurs.
In your situation I would sand all 4 pads 1mm each for starters - then take any extra material needed off on just the outer pads.

Wear a dust mask if you do any sanding on pads.
His pads were ceramic carbon fiber and dulled the sandpaper within a minute or so.
Original thickness of pads including the metal backing plate was about 17mm

[Baron Von Zeppelin]

If you use a pry bar or big screwdriver to wedge some clearance in your pads to remove the caliper - the piston should try to protrude back out a little bit on its own from some of the back pressure still in the lines - if you leave it alone and observe.

If it doesn't do that , the piston is usually hanging in the bore.
Since yours are new they might not be properly finished all the way at the back of the bore.
Thinner pads will keep the piston more into the sweet spot - in a case like that.

If they are "new" Chinese , its likely that is a culprit.
If they are newly remanufactured GM calipers it would be less likely .

edit : the lip on the caliper piston seal aids in pulling the piston back when pressure lets off.
If the piston is hanging , it can't do that .

[Cammer-6]

caliper seals play a part in retracting the caliper piston.
sticky pistons,hard seals will keep this from happening

[bobzdar]

Thanks for the pointers, I'll see what I can figure out. The reason I went with all new stuff is so I wouldn't have these issues, but I guess in today's world that's not any kind of guarantee.

[bobzdar]

I think it's the booster. I unplugged it and went to manual brakes and if they're dragging, it's vastly reduced. No more brake smell, wheels were warm after a quick 1-2 mile drive but not hot at all. Going to see if I can source a new booster locally and return the one I have, though it's 2.5 years old so unlikely.

[keith k]

Great to see it coming along!