1991 PATHFINDER Does this car have a proportional valve and would this cause front calipers to lock up?

no
rear wheel brakes operate at different pressures because of the proportioning valve in the lines
you are correct that failing brake hoses can fail internally and lock the brakes on but I feel that that is not your problem
This is how disc brakes work
There is no check valve in the master cylinder as there is in a car with drum brakes so any fluid captured by pedal ( the exception here is if the car has read drum brakes in which case that section of the master cylinder will have a check valve )movement is pushed into the calipers and applies the brakes
Now on releasing the pedal , that fluid is allowed to return to the reservoir by a compensating port in the master cylinder
That port blocks up and the disc brakes remain on.
heat build up from the brake pads expands the fluid and so pushes the pads harder onto the disc( Lock on that you release by bleeding the calipers)
SO the problem will be in the master cylinder
having the brakes professionally flushed may relieve the problem for a period but a new master cylinder fitted professionally will fix the problem

Do you have ABS? If so the problem may be in the ABS hydraulic control valve.

Very likely yes. There is a seal at front of booster which may fail if brake fluid soaks it. Only leaks when you apply brakes, and this will affect idle.

To and from the master and calipers. Did you replace the calipers also? Lock ups usually occur when a brake hose is bad or the caliper is bad/binding. Drive it and check the wheel the gets the hottest. Short drive. They get really hot, be careful( will take the skin off your finger). You can get new, bad calipers. The booster and proportioning vavles will not cause a lock up.

did you check proportion valve and reset it

You have drums in the back? Pretty sure you do. These need adjusted out to fit the drum. Or replace the drums. and adjust to fit. Old drums will have a lip to go over. Usually rust.. some metal.

Replace the rubber brake hoses

They are collapsed inside

You never do only one caliper,
you do both

Now you need to turn the rotors & new pads
Their fried

It could be but I would suspect that the left side caliper is seized and not working. This would case the proportioning valve to send the extra pressure to the right wheel, locking it up Hope this helps.

Possibly the Proportioning valve. its where the lines go after the master. it has a bleeder screw of some sort on it. The pins inside wear out and stick causing uneven pressure. Replace or rebuild it if that be the case. Also chect the master for fluid leaks at the end of the piston where it connects to the booster push rod. If its leaking rebuild or replace it. 

I suspect that you have front disc and rear drum brakes.You did not say which of your brakes are locking up. I suggest that if your front brakes are locking {skid} that means your rear brakes are not engaging. If your rear brakes are locking {skid} then I suggest you have problems with park brake or brake fluid leaking onto the rear shoes.
More info would help me help you. I will check back soon.