diagnoses? calipers, master cylinder, proportioning valve, or???

 

We can guess from thousands of miles away, or you can inspect your own brakes. See signature.

BTW, wrong forum.

 

Another good one is crushed Brake line

Master cylinder pushes fluid into the caliper the crushed line keeps the caliper from pushing fluid back

 

Sticky master cylinder. If the piston can't return fully (can also be caused by the rod between it and the pedal being adjusted too long) then the pressure pumped into the system can't release back into the reservoir. The seals work better in the 'pressure' direction, so you can still pump some fluid into the system, and if it can't get back out, the brakes get stuck on.

Also sticky rusty crappy calipers. Swollen rubber in flax lines acting as a one-way valve. Or the pads are sticking in the calipers. Etc.

 

Is it both front brakes or just one side?
both (as already stated by others) would point to a blockage common to both front brakes, like the master cylinder or a crushed/blocked line.
when they are stuck (if it is both) loosen the front brake line at the master cylinder and see if the pressure release makes the wheels spin free.
you will have to bleed the brakes after doing this and you may find something else in the process.