A master cylinder has a restricted compensator valve what problem will i most likely find with the pedal?

Did you let the master cylinder run dry? If so, you have to bench bleed it.

disc brakes can only lock up because the fluid is not allowed to return to the master cylinder reservoir
That is normally because the compensating port in the master cylinder is blocked
You say that you have fitted 2 new master cylinders but did you check the brake pedal free play because if there is not enough free play then the piston cup covers the compensating hole and the problem that you have is the result

check the adjustment of the pedal for the master cylinder
the rod attached to the pedal must be free about 1" movement before the master cylinder is activated
if the pistons in the master cylinder are not free of the compensating ports tyen hot brake fluid will push on the calliper pistons and apply the brakes
the load sensor is only a proportioning valve to allow more pressure to the rear brakes under a loaded situation
you may have to take it to an accredited brake specialist shop to properly diagnose the problem

If you are loosing prime in the system but not loosing fluid my best guess would be the master cylinder.

does the pedal have pressure then go down after valve is cracked? With a hose on the nipple to a bottle to catch the fluid, press rear brake pedal down. Don\'t pump the brakes. Crack the bleeder valve while continuing to press down on the pedal. Once the pedal reaches the limit of its travel close the valve. Once the valve is closed; and not before; release the pedal and let it return to its at rest position. Repeat this process until you have good pedal without the need to pump, good clean fluid is coming out the valve, and no air is in the fluid coming out the valve. Be sure not to let the fluid in reservoir get too low or you will suck in air and need to start over and don\'t press the pedal too quickly or you can shoot a stream of brake fluid out the reservoir.

In very extreme cases, if you have lost prime in the master cylinder, it may be necessary to bleed the master cylinder first. THIS IS A LAST RESORT. Remove the brake line from the master cylinder. Try to limit the loss of fluid from the brake line to bare minimum. Thread a nipple into the master cylinder. Run a hose from the nipple into the reservoir. Be sure the hose stays submerged in the fluid in the reservoir and pump the brake until no bubbles come out the hose. Remove the nipple and reinstall the brake line. Slowly press down the brake pedal while tightening the line to minimize the air bubble in the brake line. Again be sure not to release the pedal until the line is tight. Then proceed to bleed the brakes as you normally would. Do not pump the brakes up. Just press the pedal down, crack the valve, close the valve, release the pedal, repeat.

Test to see if the brake pressure is being released at the wheels. First pump pedal several times are the wheels locked then open bleeder screw if fluid squirts out the fluid is not returning to the master cylinder then do this again this time release pressure at master cylinder wheels move then the master cylinder residual check valve has failed (need a master cylinder) if not then the flex hoses are restricted

There is still air in the system or you have a faulty clutch master cylinder. Try disconnecting the line at the master cylinder and place your finger over the hole in the cylinder to act as a non-return valve, have someone gently press the pedal and if you cannot get pressure there and a suction when the pedal is released then replace the master cylinder as there is a blockage in the return and compensating ports.

I've found on vehicles with drum rear brakes, if the rear brakes aren't adjusted correctly, the pedal must be pumped up to get good pedal. The reason is you have to make up the distance from the shoe to the drum before the pedal builds pressure in the system. If the problem comes back check the adjustment again. The brakes can back off when the ratcheting wheels get worn. Self adjusting brakes can be worse. Worn/loose front wheel bearings can push pads back and cause a similar mushy pedal.
Check for leaks, check adjustment, bleed before condemning any parts.
Bad booster or pump=rock hard pedal. Bad master cylinders usually won't hold a pedal. If it pumps up, it will sink. The compensation valve usually fails with a leak as do wheel cylinders.

alot of new cars cannot be bleed correctly without a scanner to open the valve in the ABS motor. You may have to have a shop perform this job

bleed it from wheel to wheel the one fartest from the cylinder andworking away around till yo are at the closest, you need a quart of dot 3 brake flid to do it and two people to do it, pump the pedal three times ad release the cylider valve and do so til yo get clear brake fluid do this which each wheel and keep filling the master cylinder, do this twice if it's working the pedal will keep rasing. if the pedal becomes sollid and still not all the way up then adjust the rear brake start wheels where te wheels barely touch the drums, aslo there is a compensating valve on the rear axle remove the nutthat holds it to the bed and open it wide open and continue to bleed, that should fix you up.