1995 Buick LeSabre Limited - Excessive Brake Pedal Travel Post Repair

[fan of four doors]

Good evening everyone,

A bit of background on what's going on:
My car blew a brake line around Thanksgiving last year and had sat till about 3 weeks ago. A friend is helping me repair it.

To summarize what's been replaced on the car due to wear & tear:
-Brake master cylinder
-All hard metal hydraulic lines
-Front calipers & pads
-Front rubber brake hoses
-Front rotors
-Rear wheel cylinders
-Rear drums
-Rear shoes
-Rear brake hardware
-Rear rubber brake hoses
-Proportion valves on rear lines

We made sure to bench bleed the new master cylinder to evacuate any air in it both off and on the car.
The ABS activates in gravel and does work. It is not a model that requires the auto-bleed procedure to be run on it to bleed it.
A scan tool was used and read no ABS codes.
The brake power booster holds vacuum and has no leaks.
There's no leaks anywhere else that we are aware of.
All of the 4 corners have had the brake lines bled from RR, RL, FR, FL.

And yet here is the problem: The brake pedal will stay firm with the engine off and operate seemingly as intended. Once the engine is turned ON, the pedal will travel to just about all the way to the floor. The brakes still work though, and the ABS will activate in that state.
It does not feel like it should be traveling that much.

Does anyone have the Factory Service Manual for this Buick or an idea about what is going on that we haven't considered yet?

 

[cjpayne]

I would try putting back the old master cylinder, just to see if the new one is flawed.

 

[fan of four doors]

I would try putting back the old master cylinder, just to see if the new one is flawed.

Unfortunately that one's gone back to the DC for the parts store when I did the warranty swap out. However, the same problem existed with the old & new M.C. Thank you though.

 

[Homer Jay]

I believe there's still trapped air. Don't know what procedure you used, but carefully bleed the system again.

 

[fan of four doors]

I believe there's still trapped air. Don't know what procedure you used, but carefully bleed the system again.

That could be it I guess but we have bled the system so much already. Even managed to secure a scan tool to run the "auto-bleed" program. The pedal is firm when the engine is off, but as soon as the engine is turned over it resumes it's softness and excessive travel.
Will bled it more though, but I really don't think there's air trapped in the circuits.
Thank you!

 

[ulycyc]

Make sure the rear brakes are adjusted by the emergency brake cable. If the cables are frozen going into the backing plate the shoes wont adjust and you will get low pedal. Most of the time its the right rear cable frozen and or cable housing broken that causes the issues.

 

[cjpayne]

Please post when you solve the problem. I've never seen this before.

 

[fan of four doors]

Make sure the rear brakes are adjusted by the emergency brake cable. If the cables are frozen going into the backing plate the shoes wont adjust and you will get low pedal. Most of the time its the right rear cable frozen and or cable housing broken that causes the issues.

I will keep this in mind with when we revisit this car soon.