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First time here and hoping for a solution but willing to take suggestions.
I've got a real "head scratcher" for you guys.

07 Impala (VIN: 2G1WB58K079104049)

After 8.5 weeks at the Dealership they are throwing in the towel on why the brake pedal goes to the floor. BTW, we went through 3 different field service engineers at the dealership, a shop manager, and a couple of service mechanics too.
This all started when my son came home from school after his spring semester and we changed the pads and rotors. We used OEM parts except for rear rotors due to availability. Bled brakes (two man) several times (RR, LR, RF, LF) but brakes were intermittent - good one minute but pedal to the floor the next. It acted as if it either still had air in the system or there was a loss of pressure or fluid leak but absolutely no leaks ever appeared. We then changed the master cylinder (aftermarket) after sufficiently priming it prior to installation. When the same problem persisted we brought it to the Dealership.

The Dealership diagnosed the problem as a faulty brake booster (aftermarket) and changed it. No change in the brakes. No codes with the scan tool from the Dealership. They repeatedly bled the brakes with the scan tool to activate the ABS to remove air and air would come out each time and brakes would work fine. But the next morning the pedal would go back to the floor and there would be air in the system.

It is puzzling how air can enter (whether passively or actively) the system without fluid leaking out. To me, this defies logic and I do not believe air can enter the system passively so it would seem that air is being actively introduced somewhere. Could this occur between the brake booster and the master cylinder. Is it possible that the booster can push air into the master cylinder and somehow completely sealing (like a check vale) upon recoil and not allow fluid to leak out when brakes are disengaged?

If you can solve this riddle you should promote yourself from master mechanic to Jedi.
 

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FWD= sorry, wrong place. But my guess is 1) dealerships have ZERO idea what they are doing today. 2) They put on a part from China, so it's probably garbage anyway. Don't pay the stealership anything and take your "car" to the best private mechanic in town.
 

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Have a look at this thread:
It reads just like your post: Dealer changes new master cylinder then gives up because this did not fix the problem.

Short version: Many different vehicles can have the symptoms of pedal to the floor an/or mystery changes of brake fluid level with no error codes.
A ABS valve may be leaking allowing the master cylinder's pressure to be leaked off to the ABS reserve or back to the master cylinder reserve.

The ABS valves are the place to look if a master cylinder has been replaced and the pedal still goes to the floor sometimes.
 
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