1965 Valiant low brake pressure

Some basics:

The two big reasons for a LOW pedal are loose brake adjustment or air, which should result in a soft low pedal

When you adjust shoes, adjust them up "no can turn" and back them off. This will center/ seat the shoes

The master piston "may be wrong one" must have the correct stroke to "not" have too much free play, yet allow the piston to come back past the port in the master. Easily seen by fluid squirting back in the return, easily felt with a finger or two on the pedal "feeling" for free play/ start of stroke.

On older systems, (single) they should ALL have a "residual pressure valve" and in those masters, they look like a "salt shaker" lid in the end of the master --far end of the piston return spring, and usually the "first thing in"

Make sure your wheel cylinders AND the master are the same / correct bore size

Since you are having a low pedal problem, I would tighten up all wheel adjustments until the wheels won't turn, then bleed the brakes and VERIFY that you have a very high, very firm pedal at that point. Then back off the adjustments until they drag. At this point as suggested above, "stomp" the pedal to recheck adjustment/ seat the shoes. Most brakes will still scrape after adjustment for awhile. In other words, do not back them off too far.