If the pedal does NOT get hard with multiple pumpings;
in all likelihood the compensating port is not operating correctly;
or the master is bypassing fluid back into the reservoir,
or, as mentioned, you could have air stuck in the master somewhere.
There is also a slim chance that your new brakes have a mechanical issue.
As to diagnosis; I would
remove the calipers and C-clamp them into the bottom of the bores, then check the pedal. If still soft;
you will have to check that the C-ports are open and the pistons inside are returning to the park position,all the way towards the firewall.
After that, if still soft, I would "bench-bleed" the master, then pressure-test it.
My gut feeling is that the pistons in the M/C are not parking at the back, or the C-ports are restricted/closed. When this happens, the same brake fluid just shuttles back and forth in the lines with every foot pump, so the pedal cannot get hard unless it starts hard. The C-ports are there so that after every pump, as the pads wear out, more fluid can enter the line, to "compensate" for the wearing friction surfaces. In a properly working system, rapid multiple pumps will trap the fluid in the lines with a lil more each time, and after a few pumps, the pedal should rise and become hard. When you release the pedal, the relaxing calipers and shoe return springs will force the excess fluid back into the M/C thru the C-ports, and the pedal returns to normal.
Reasons for the C-ports to not return fluid are;
1) it never went out in the first place; like air in the master.
or piston seals leaking allowing the fluid to bypass during application
2) incorrect push-rod length
3) pedal not parking at the top
4) pistons sticking in the mud/rusty bores
You will know that the C-ports are open, by;
a) the fluid roils when you push the pedal down. And
b) the fluid roils again when you return the pedal to the top. and
c) you get a lil geyser after multiple pumps. But be careful; if there is air in the lines, the geyser can be two or three feet high! depending on how many pumps you did, how much air is/might be trapped in the system, and how far out-of-adjustment the rear shoes are; I ain't buying you a paintjob!
