[mr48chev]

I'm taking it to believe that "hard Braking" means that he is running up the street and then laying on the brakes panic stop style.

Can you pump the pedal up after that hard stop if you pump it slowly several times? If the pedal comes up when you pump it a few times there is air somewhere. if it doesn't come up you have another issue.

Residual valves keep the pressure in the lines so there is pressure against the seals in the calipers and cups in the rear cylinders. You do have them installed in the correct direction ?
Is your free travel on your pedal adjusted correctly?

If the pushrod is adjusted too loose you may be running out of pedal travel just as you would if the drum brakes aren't adjusted. How much free travel do you have with the pedal? You want about 1/8 with the pedal all the way up

The late Dick Spadaro who was a well known vintage Ford guru gave these instructions for adjusting the pushrod.
Without explaining just what parts you purchased new that resulted in a different pedal experience makes trouble shooting difficult. Since you do have brakes but extensive pedal travel its more likely the problem one of these areas. If you have 1/4" free play in your pedal to master cylinder push rod you have way to much free travel. If your new pedal assembly pushes from the pedal to actuate the master cylinder via a clevis or heim adjuster you should re adjust the pushrod length.To get the free play necessary to prevent constant pedal pressure you should remove the bolt holding on the connecting clevis, pull the pedal to the up position desired. With the push rod inserted into the master cylinder and just touching the piston, adjust the push rod clevis so that the bolt hole opening in the clevis exposes 1/2 opening of the mounting bolt hole in the pedal arm on the master cylinder side.. Replace the clevis bolt and check the pedal to master cylinder free play, should be just a little 1/8" or so. Now just static brake check the distance the new pedal adjustment travels to the floor. If this is the same as before you have have not purchased a master cylinder with the same displacement as the old one as posted Mike 51 suggests. It is important to match the old displacement bore with the same displacement bore on the new master cylinder.

On the master cylinder bore he mentioned in that answer to a question the mc bore should be compatible with The bores of the calipers and wheel cylinders but I don't think that is the issue with your setup as those aftermarket brake setups usually closely match the metric GM calipers and matching rear wheel cylinders.
How did you adjust the rear brakes and how tight are they adjusted? I was taught to tighten the adjustment until you can't turn the wheel by hand and back the adjustment off 11 clicks. That works real well on the older axles with the adjustment slot in the backing plate but getting them right on brakes that don't have the slot is a challenge. My rule of thumb with that style is to take them out so I can just get the drum to slide over them and may have to help it a bit. Properly set, the shoes will have a bit of friction against the drums at all times.

I should note that in the 70's between the time I was in the Army and the time I started teaching I worked as a front end mechanic/ brake mechanic for several years and did a lot of brake jobs. Mostly on drum brake cars in those days. I had to deal with a lot of brake issues back then.