73 Drum to disc prop valve needed?

Many manual 4 drum systems didn't have a true proportion valve. They had a basic distribution block with the warning switch along with residual pressure valves in the master cylinder. So if you didn't change the master cylinder on the 70 model, the difference between that master cylinder and the master cylinder you have today ( lack of residual pressure ? ) might be where the squish comes from. Just spit balling here since I don't know what year models had a true proportion valve. From what I've read, most owners buy a new aftermarket proportion valve, adjustable type or not.