Sinking brake pedal to floor

[86 GN DDR]

I recently changed over to vacuum brakes. I installed new brake lines, new proprotional valve, new master cylinder, booster, and vacuum block. I started to experience a sinking pedal. My mechanic stated that it sounds like a bad master cylinder. I changed it out yesterday. All was good. Then started experiencing the same sinking pedal. Just wondering what it might be. Any advice would be helpful. Bad booster?? Not enough vaccum??

 

[TexasT]

Id say you have another bad mc. The booster just delivers boost to make braking and pedal easier to operate. If the pedal isnt staying firm it has a leak somewhere that isn't holding the pressure you are delivering by applying the pedal. That leak could be internal, or a hose or wheel cylinder. With the later two you will see fluid escaping.
Maybe try s different brand of mc or a different part house.

Posted from the TurboBuick.Com mobile app

 

[87geeinn]

1) Did you bench bleed the MC before install? 2) Did you bleed the brakes? Stupid questions but I didn't see any mention of it.

 

[chopped39]

Try doing a line lock test. get pinch off pliers and install one on each hose and then see how the pedal feels. If the pedal feels good have someone remove the pinch off pliers on at a time .If the pedal drops excessively when removing one there's air in that part of system. very easy test. Like stated above the master has to be bench bleed before installing.

 

[TurboBuRick]

Does the pedal go down and get stuck?

I had a bad booster once that would apply full power to the bakes and would not release pedal until vacuum was removed.

Replaced the booster and all was well.

Rick

 

[86 GN DDR]

Thanks everyone for the advice. I swapped out the mc..Bench bleed the mc and readjusted rear drums. ..No more sinking pedal and better feel.

 

[87geeinn]

Thanks everyone for the advice. I swapped out the mc..Bench bleed the mc and readjusted rear drums. ..No more sinking pedal and better feel.

Also, keep in mind that you have a stepped-bore MC and those take considerably more patience and time to bleed. The quick take-up valve tends to trap air bubbles versus a standard bore MC. It took me quite a but of stroking (lol) to get all the air out of my MC when I swapped a new one in. Tapping on the outside of the bore and using one of those plunger tools to force fluid up through the ports will help but otherwise, just keep stroking. And when you think you've got all the air out, keep stroking some more.