1985 Ford F-150 mushy brake pedal

I have replaced the vacuum assist, master cylinder, both rear wheel cylinders, and both front calipers along with all new rear shoes, and new pads for the calipers.
After first priming the new master cylinder, all the lines were bled out starting at the right rear, then the left rear, then the right front, and last the left front. After all the air is out of the lines, the pedal is solid and the truck has good solid braking action.
After the truck has sat for a few days, the brake pedal engages the brakes right away when depressed, but then the rest of the pedal beyond that is like stepping on a soft basketball, and the braking is very poor!
I have inspected the entire brake system from front to back and see no evidence of any leakes anywhere.
After bleeding the entire system again the brakes will be good, and again after the vehicle sits the pedal will become soft again.
There are no anti-lock brakes on this truck, so I should be able to leave skid marks, but the brakes will not lock up! This has been driving me crazy!