74 Chevy K20 brake issues!!!!

bam_bam

New Member
Joined
Aug 29, 2011
Location
salisbury nc
I have a 1974 Chevy k20 with military dana 70 axles. I have rebuilt all the brake lines and rebuilt all the brakes and wheel cylinders. Truck has a brand new Power brake booster. I know the master cylinder and porportioning valve are both good. Heres my problem. When i bled the brakes with the truck off i get a good firm pedal and the brakes are working. If i start the truck i loose my pedal but the brakes still work once the pedal is to the floor. I have tried bleeding them while the truck is running and nothing i do stiffens the pedal back up. I know i'm getting good vacum from the manifold. If anyone has any suggestions or sees anything i'm missing or not doing please let me know. At a stand still on this build until i can figure this out.
 
sounds like the booster is bad. if you were closer i have a free one from my build that you could take
 
Probably have to bench-bleed the master cylinder if air got in it when you replaced all the hard lines. It is not a huge deal to do just a pain to pull all that stuff apart. I had to do this on my Blazer when I replaced all that stuff years ago.

If the booster was bad but the hydraulics were solid then you would not get a pedal dropping to the floor, instead it would seem like the pedal effort was super-high since the booster would not apply extra force to the master.
 
What method are you using to bleed the brakes ?
Also did you adjust the brakes before you bled them?
The best method to bleed brakes is have someone in the truck and you open the bleeder then have them push the peddle down slowly when the peddle is at the floor close the bleeder and repeat .I usually do this at least 4 times on each wheel . On the older and some newer chevy's the old pump and hols method does not get the all the air out.
 
Had somewhat the same problem in my chevy, I just had someone pump the peddle while I cracked the lines at the proportioning valve and then moved to the furtherest brake away from it, then bleed them like normal.
 
Aight i'll give the valve a tap and try seeing if i have trapped air there. I went ahead and got a new master cylinder anyway and bench bleed it upon install. Now upon engine start the pedal will get firm after 3 pumps then fall to the floor. So i'm getting closer to figuring this out. I've done every bleeding method i know know before i posted on here. Only thing i havent tried is breaking the lines at the valve. Thanks guys everyones input has helped a ton.
 
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