Brakes - 1st pump nothing
 

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?

Re: Brakes - 1st pump nothing
 

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

Re: Brakes - 1st pump nothing
 

Ask a professional, but I think there should be a residual 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.

Re: Brakes - 1st pump nothing
 

Sounds like a bad master cylinder.

Re: Brakes - 1st pump nothing
 

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.

Re: Brakes - 1st pump nothing
 

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.

Re: Brakes - 1st pump nothing
 

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

Re: Brakes - 1st pump nothing
 

Yes it is, I have a special tool to do this.

Re: Brakes - 1st pump nothing
 

If I'm remembering correctly most stock MC's have the residual valve already installed in the rear brake line outlet. Is your MS a factory style one made for disk\drum or did you go with an aftermarket\performance unit? Could be a bad valve but the problem of the first pump not having pressure definitely sounds like either its not holding pressure or the MC is just straight bad.

Re: Brakes - 1st pump nothing
 

MC is a "Corvette style" kit with the power unit from the Right Stuff, made for disc/drum. I have heard that some MCs or prop valves have a residual valve in the rear of the unit. I have changed the prop valve just "chasing", no difference.

Re: Brakes - 1st pump nothing
 

The info I posted above was from the MBM brakes website. https://mbmbrakes.com/brake-pedal-diagnostics/ They have some good info.

You shouldn’t need the residual valve but it might fix the problem. It's possible the springs on your drums are stronger and the residual valve helps keep some of the pressure on the brake shoes.


I had a really hard time finding my brake problem when I switched to 4 wheel disc, issue was the check valve on the booster. You could check that, air should only pass through one way when you blow on it. Unlikely that's your problem but it's an easy thing to check.