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hello , i have a 1939 chevy 1.5 ton truck , updated to all 4 wheel disc brakes with a single 7 inch brake booster master cylinder combo 1 inch bore mounted under to floor board from speedway motors . the vacuum line is hooked up to the intake manifold small block 350 . i have bench bled the master cylinder 2 times and 1 time in the truck , installed 2 lbs residual valves 1 for the front and 1 for the rear, i have bled all 4 ports starting with the farthest .I can not get any pedal only if i pump the brakes 3 or more times then they are spongy . while the master cylinder is still in the truck , i closed both ports of the master cylinder and stepped on the pedal , good firm pedal goes down 1/2 way to the floor . i have jacked the truck up 1st the front bled the brakes and jacked up the rear and bled the brakes making sure the master cylinder was the highest point in the complete system . no leaks any where in the system . any help ? thank you for looking at this thread . 1pete1

Spongy means you still have air in the system, low peddle or having to pump it could be the calipers are not floating very well and the pads have not contacted the rotors OR the master is not big enough for the volume of the slaves.

1Pete1,

I’m helping a friend with a 50, 3100, restomod that is having the same issue. He too purchased the components from Speedway, including the disc/disc master cylinder. We bench bleed the master cylinder, bleed the system starting at the farthest point numerous times with no improvement. Speedway replaced the MC under warranty and we started over again but did not solve the problem.

Has anyone experienced a bad 2lb resisidual valve? Like your system, first pump nothing, third pump hard pedal that doesn’t fade. I know this is probably improbable but, I’m wondering if the residual valves (Speedway brand) front and rear, are not holding the 2 psi on the calipers?

I’m not knocking Speedway, just looking for a common denominator.

Do the calipers have the bleeder screw pointing up?

Well you can always plum it without valves and test it out. Residual valves are purely a safety feature to keep fluid from
Back draining and introducing air with master below calipers.

And I had same spongy issue first post had every time air in lines. And it does not take much. Once I vacuum bleed the lines and taking my time spongy went away. It later became a issue again. It was determined the rear hose was weeping but not enough to drip unless a lot of braking. Replaced hose issue resolved.