Be sure master cylinder on the firewall has been replaced or completely cleaned and rebuilt with new seals.

Be sure push rod adjustment between pedal and master cylinder is correct and linkage is not binding.

If the power brake unit has a residual check valve, the master cylinder must not have a residual check valve.

To test - with the brakes locked up, loosen the brake line at the master cylinder outlet. If this releases the brakes, the trouble is in the master cylinder, push rod adjustment or pedal linkage.

If this does not release the brakes, loosen the hydraulic brake line that goes to the wheel cylinders (located on the end of the slave cylinder of power brake unit.) If this releases the brakes, the trouble is in the power brake unit, wheel cylinders or brake shoes.

Loosen nuts holding master cylinder to power booster. If this releases brakes, the push rod between the booster and the master cylinder could be out of adjustment.

Remove master cylinder. If adjustment screw is present on the rod, lower the screw about 1/16" at a time until it does not apply the master cylinder piston.
Caution - If screw is turned in to far, it will cause excessive pedal travel and cause inefficient brake performance.

If no adjustment screw is present, check pedal linkage for binding.