I just upgraded the front brakes on my 86 pickup to V6 rotors and calipers, and a larger bore MC. I bench bled the MC per the manual, and did a full system bleed on the truck as well. I still have a very spongy pedal though. With the engine off I can build up pressure on the pedal, but it takes a good ten pumps to get a firm pedal. I can hear a slight humming sound when I press the pedal though...? Is this a sign that I need to replace my brake booster?

I also have a feeling I have a bad wheel cylinder on the driver rear. I plan to do a full rebuild on both rears when funds allow though.

my main concern right now is the humming noise though...

what do you guys think?

 

What makes you think you have a bad cylinder? Leaking? It's easy enough to pull a shoe & check.

Not sure what to tell you about the humming sound....

 

well the wheel cylinder checked out... thought it was leaking but it was just playing tricks on me.

As for the humming sound, I had my dad press the brakes while I listened. It is coming from the vacuum line that runs to the booster.

I still have the same problem though. Absolutely no pressure on the pedal. You can build up pressure by pumping the pedal for a little while, but as soon as you release the pedal it goes back to mush??? I am at a loss right now fellas. Anybody had a similar problem?

I know there can't be any air left in the brake lines(at least I wouldn't think there would be). I flushed the lines until all I got when I bled them was nice, fresh, clean fluid...

Thanks in advance
-Jason

 

your brake booster will only work when the engine is running if u have a hissing with the engine off then u ether have air trapped in a line of bad master cylinder...
if it is hissing when the engine is running then u have a bad booster. but you would have a veer hard peddle with in 2 pumps if there is no air in lines... to me it sounds like the master cylinder i got 2 bad ones that where brand new