I have a 1993 Z71, (owned 15yrs). Everything in the rear brakes was bad. Converted over to rear disk brakes. That part was easy. My brakes have been spongy for years. I replaced master cylinder (NAPA brand). I have bled and vaccuumed the system and brakes are still spongy. With truck (not) running brake pedal is on top like it should be. As soon as i start the engine pedal drops to the floor and is spongy. Yes master was bench bled. I have bled brakes for years, but have never ran into this problem. Any help would greatly be appreciated. Thanks. I know someone on here has the answer.

 

Maybe the brake booster?

 

sounds like a bad booster to me.....or it could be the check valve in the vac line to the booster (if it has one)

 

Thanks i will check the booster out. Will let you all know what i find.

 

I checked booster the way it was explained to me. See if you all agree. With engine off pushed pedal several times to release pressure. Then held pedal down slightly and started engine. When engine started the pedal went down . Do you all agree this is a proper test.
I also read in the shop manual for GM that on the combination valve the small pin in the end has to be held (in) before proper bleeding can be accomplished. Does anyone know if this is true?? Thanks.

 

well, as far as the booster goes ive never been fond of ANY of the tests that there are for them...but i would say that the booster "tests" good...however this does not mean it is good...as far as the combination valve goes....yes that is the correct way to do it.

 

Did the one booster test. Checked ok. Finally got hold of a buddy that used to work for the local chevy dealership. He had me plug both outlets coming out of the master cyl. Started truck and pedal did (not) drop at all. He said this would test both the (even tho it is a new master cyl) master cylinder and the booster. If this is a good check then both are ok.
There is only 2 other things in the supply lines. The combination valve which both front and rear lines go thru. Then rear line leaves combination valve and goes thru the dump valve. Well now another dilemma. Which one is the culpret????. Ebay has combo valves cheap compared to what the GM dealership wants ($271). Anyway any input on how to check either on of these is greatly appreciated. I promise i will post pics of the disc brakes conversion as soon as i solve this problem. Maybe my problem when it is solved can help someone else. Or should i just buy a combo valve and stop all this testing that no one, not even GM will guarantee will find the problem. Thanks Guys.

 

a bad proportioning (combination) valve could be the culprit....have you checked..........(i know bad word)......aftermarket...for the valve?....also a rubber line that is starting to collapse "CAN" cause some of these same symptoms

 

How would i check the combo/proportioning valve?? And the only 2 lines i have left that have not been changed that is rubber is the 2 front lines. Thanks.

 

That era truck had an issue with the abs module making the brakes spongy.