Need help troubleshooting problem with brakes

Hello, I just picked up a 72 Duster and am having some problems troubleshooting the brakes
(All 4 are disc brakes)

Here's my issue:

The brakes are mushy, and the brake pedal doesn't travel very far when pushing it all the way down
When I completely depress the brake pedal, the car just comes to a stop while on the slant of my driveway

I bled all 4 brakes today, checked for any leaks in the lines
Bleeding the brakes improved them a bit, but not to the point where they are working good
I checked the seal on the master, looked to be fine
When I pump the brakes while the car is off, the pedal stiffens to an extent, but doesn't go completely solid
And then after starting the car the pedal slowly and slightly unstiffens

If anyone has any suggestions on what I should look at next, that would be extremely helpful
I ran out of time today but tomorrow I plan on looking to see if the calipers aren't moving properly, or need to be lubricated.


Any help/suggestions are much appreciated
Thanks