Here's the story:

I recently swapped to new Brembo rotors/Hawk HPS pads/Stainless steel lines/ATE super blue/929 master cylinder/rebuilt front calipers

I bench bled the 929 master before hooking it up, bled all 4 corners according to the FSM, and made sure there were no leaks. Here's my problem:

At any sign of pressure in the brakes, the front calipers stay stuck on the rotors, rendering the wheels extremely difficult to turn by hand. This "stuck" phase resides as air somehow ends up back into the system, that is, after maintaining pressure for a matter of 30 secs or so, air is back in the system and the brake pedal will go to the floor.

I dropped off my car at a rotary shop to get the issue resolved.

They replaced my M/C lines: Same problem.
They put my oem 15/16 M/C back on: Same problem.
They bench-bled both M/C's again: Same problem.
They re-bled the M/C and brakes countless times: Same problem

Everybody is stumped, and at this point, I need a brake shop specialist to find out what's going on unless some of you have some ideas. Thanks!

 

Ok, if I understand correctly:

- you depress the pedal AND release it... the front pads still exert force on the rotor

- the force remains without the pedal being depressed

- when the rotor is finally released, there is then air in the system...

Does this happen with booster vacuum and without?

You can eliminate the master side of the system as the cause by doing this:

Depress pedal so that the front rotor is stuck.
Immediately go to stuck wheel and loosen the bleeder screw (watch your eyes, stuff is nasty)
If the rotor is unstuck
-you know that the caliper/pad is not binding
-something on the master side (really anywhere b/t caliper fitting and pedal) is maintaining pressure or not letting fluid return from the slave side back to the master

If the rotor is still stuck
-the master side is not the problem and there is a problem with the caliper

I can't explain how air is returning into the system without any fluid loss while under pressure. It's a two way street. If you aren't leaking fluid, I am skeptical that air is being drawn back into the system unless there is no fluid in the MC.

 

Very good and clear explanation on this diagnosis.

I couldnt have said it better myself.

-AzEKnightz

 

I dont think there are air back into the system. More as the fluid were used at the stuck caliper causing the brake pedal to drop to the floor. Air in system = spongy.

However I am leaning towards a bad seal or the seal in the master cylinder is not positioned correctly therefore causing this problem.

-AzEKnightz

 

Alex, you hit the nail on the head in regards to what I was trying to explain. I'll try your method to try and rule out the calipers/master. It's weird, because no fluid is leaking - this has been verified by both myself and the shop, yet, somehow the pedal (after being pumped) will only maintain pressure for less than a minute (during which the calipers are stuck), and then the pedal sinks back to the floor again?

 

I guess I get lucky sometimes

OK, but is there really air in the system? Or does it just feel like there is air in there? I ask because a pedal can feel spongy, drop to the floor and act like you are pushing on air without there being air in the system. I've experienced this with poorly adjusted drum brakes especially, but it may apply here to a malfunctioning caliper

Also, it's sometimes hard to tell if you are actually pushing air out during bleeding unless you have nice equipment because the bleeder tube can empty out and fill with air between bleeds.