I put together some front discs on my 75 f250 4wd, using chevy 1/2t calipers. I removed the residual pressure valve from a fresh MC. Pedal is low and soft, and consistent, but not spongey, but braking isn't very efficient. Pumping the pedal doesn't bring it up. Brakes are bled, rears are adjusted, I'm thinking I need a different MC, maybe one with a bigger bore. Do the disc brake trucks have a larger bore MC? Any recommendations? Thanks in advance.

I would pick up a MC for a 77-79 truck. They have the correct valving and piston size for disc brakes.

Looked at a couple of MCs at autozone, the 79 F250 4x4 MC had the right mounting pattern, fitting sizes, and a bigger piston, didn't measure the pistons but looked to be .25" or so bigger. I had to adjust the pushrod out a few threads. Stopped better than ever before, the bigger displacement made a big difference.