73-76 sliding calipers not sliding?

The caliper only needs to move now and then, to compensate for padwear.
Seal retraction will pull the inboard pad away from th rotor, and rotor wobble will pop the outboard pad back.
But non-the-less, the ways should be smooth, and the loaded calipers should slide back and forth with the pistons pushed back, and the retainers fully secured, on dry ways,with just a bit of effort. The key is that the pistons be retracted.
Op you know what seal retraction is right? That's when the square section O-ring seal, inside the caliper, relaxes after an application and returns to it's former square shape. To do this, the piston has to be free to move in it's bore. That means no rust/no sludge inside the bore,and no rust/gunk/gravel/etc in the receiver groove for the sq. section O-ring. The dust boot is pretty good at keeping that stuff out from the atmospheric side. But when new pads are installed in old calipers, that's when the trouble starts. To get the new pads to fit, the pistons have to be pushed back into their bores.Where the rust and gunk have been accumulating. But often overlooked is also the oxidation that occurs on the outside of the pistons,under the dustboot, and which gets jammed into the sq O-ring during this resetting process. This External problem can prevent seal retraction from properly working, and can also lead to leaks.
And don't forget to pump the brakes up to hard pedal before putting the tranny into gear for the first time.