Brakes - 1st pump nothing

[72Mountaineer]

64 C10, front power disc conversion, Camaro rear end w/drum brakes, everything new (power unit to caliper/rear cylinders), rod from pedal to power unit moved down 1", all brake lines bled out the ass. First pump goes to the floor, 2nd pump is good. WTF?

[SkinnyG]

Are the drums adjusted correctly? They should slightly drag when turning the drum.

[bhap]

Ask a professional, but I think there ishould be a redidual pressure valve to keep the shoes closer to the drums so your first pump isn’t “empty”. They might be incorporated in disc/drum master cylinders or proportioning valves - someone with better knowledge can answer.

[theastronaut]

Sounds like a bad master cylinder.

[jweb]

I would do what was recommended above and check the adjustment on the rear drums, this seems to be really common.

Did you bench bleed your master cylinder?

Even with the master cylinder on the firewall they recommend a residual valve.

Drum brakes require the use of a 10 Ib residual pressure valve in the line. This residual pressure counter balances the drum brake spring tension keeping the shoes close to the drums. This results in a higher firmer pedal. You can test this by clamping off the rear hose removing the rear drums from the system. Now test your pedal. If the pedal gets better you will need to splice a 10 Ib residual pressure valve into the rear line.

[72Mountaineer]

Originally Posted by SkinnyG

Are the drums adjusted correctly? They should slightly drag when turning the drum.

Yes, the rear drums are adjusted correctly, maybe even a little tight. I've been doing this stuff for 60 years, and it all points to air in the system. Yes, I bench bled the MC. Bled the system more than once (ask the wife, shes an expert over the years at pedal pumping), even used a pressure pot, no change. When you say clamp off the rear hose, do you mean the hose over the rear axle? I have never heard of the fact of all drum brakes need a residual valve. I have set up and replaced many older systems and have never used (or seen) residual valves in a stock system. I set up my '53 Chevy with residual valves, as is required of a MC lower than the calipers, and the brakes worked better without them. But I've never even heard of putting valves in a system with the MC on the firewall.

[AcampoDave]

Is the MC pushrod adjusted so that the piston can fully retract to the resting position when the pedal is released.