Sounds like the main master cylinder piston cup seal/s are bad.
1948 - 1951 Chevrolet Truck Shop Manual You can
Replace or
Rebuild Enlarged.
And fix the mild fluid leak at that 1 rear wheel cylinder.
After you pump does it hold or does the pedal eventually start to sink?
Could be air in the lines, but not likely.
If main master cylinder bore is pitted your better off to replace.
