Manual Brakes Spongy Pedal

[AintQik]

Not on my PT ride, but the thinking should be the same.

Trying to trouble shoot a 50 Chevy I'm working on. Front brakes are metric calipers off like an 86 Caprice. Rears are stock drums from a Torino 9". Master cylinder is a manual 15/16ths from a Mustang II, lol I know work with me here.

Brakes are bled out fine and work, but the pedal is a bit soft for my liking. If you pump them it only gets marginally stiffer, but still soft. System has residual valves in the front and rear lines.

What, if anything can I do to try and get more pedal feel? Increase or decrease bore size on the MC?

[NOGO]

You should run a combination valve with the disc/ drum setup.

[MonzaRacer]

A few other things to check, make sure your drum shoes are adjusted up, and self adjusters are working.
From being a mechanic, I adjust the rears up till I have a decent 2 handed drag all the away around. The drum should still slide off with difficulty but not hang on the shoes AND have some drag. If it wont come off your drum may be bell mouthed.

Also make sure the drums ARE NOT over size. If its max diameter is 9.560 and they measure over that your going to have some issues.
Reason I know I have a 78 C10 was factory diesel, has larger rear drums, one day brake pedal low, light came on with every pedal push, replaced master cylinder, not fixed, replaced drums (they were over by like 1/8" over size and shoes were only like 1/3rd worn out.
Actually a good adjustable prop valve works better rather than a combo valve, this makes them adjustable.