After hard braking [emergency stop] the brake pedal was very soft and would go to the floor. Brakes still worked somewhat. Have now replaced the master cylinder and bled the system completely and the problem still exist. My mechanic has another master cylinder ordered, however there must be some other problem. HELP

 

Didyou find out the problem?

 

Just did the same thing. replaced master cylinder after bench bleeding bled all 4 wheels. Did it twice thought the first one was defective. Same problem.
I heared a whoosh when bleeding left rear both times. didn't dawn on me that was a clue. I dropped the car off to mechanic last night.Took the key to him this morning.
It snowed last night. He said go drive it and see if it will go into antilock mode. Hit the brakes antilock growled pedal poped up. Problem solved. I saw somewhere
a GM mechanic said they will sometimes reset on a gravel road if you can get it to triger the antilock. My mechanic friend said Never replace master cyl. on something with antilock
without haveing a scanner go thru an auto bleed sequence. Good luck.

 

Now I just need to learn how to perform rest of antilock brake system lol