problem with vacuum brakes - '78 Z-28

[at7we2]

I'm helping a friend of mine get his '78 Z-28 back on the road. Engine was running fine so we started with the brakes. Replaced *everything* except hard lines and proportioning valve.

Brake pedal feels nice and firm with engine off. However, start the engine and brakes are only good for one or two pumps then they go to the floor and almost NO brakes. I've done this kind of brake work before with no issues. I've never seen this problem before.

We've been through two rounds of bleeding the brakes the with the two-person method. No air in lines out at the wheels.

Any help is appreciated.

Thanks,
Rob

 

[turbojd]

Sounds like theres still more air in the system Rob, did you bench bleed the master cylinder before you installed it?

 

[at7we2]

turbojd -

thanks for the reply. problem seems to be solved. went back over there this afternoon and bled the SHEEE-OTT out of the brakes the old two-person way, this time with the engine running. ended up pushing out a few more bubbles at each wheel and that seemed to do it.

I've done probably a couple dozen major brake jobs like this where I replaced everything or almost everthing and I've *never* had to bleed a car this much!

and uh.....no.... I didn't bench bleed the master :redface: ....several years ago I developed a bad habbit of not doing that and all my master cylinder replacments have gone fine since I quit bench bleeding. what I usually do is fill up the resevoirs and let it sit overnight with a bleeder valve or two open. this come back the next day and slowly pump the system a few times then proceed to conventional bleeding of the rest of the system. been working fine that way until my friend's '78 Z28

anyway, brakes are great now and my buddy managed to get the car inspected today so he's happy. now on to all the other stuff

my next major task on this car is to convince my buddy to yank that old slow Chevy 350 and replace it with an LC2! :biggrin:

Thanks for reading and replying!