Brake lock-up 1985 honda accord brakes lock-up when hot. Already replaced master cylinder (new)/ power booster (used)/ proportioning valve (used)/ calipers (reman.)/ flushed & bled system.

If it goes to the floor and no brake warning lites or anything comes up on the dash, check the brake booster--engine vacuum and vacuum to the booster. Just my opinion. The master cylinder usually bolts to the booster.

Master Cylinder (Brake System) :
Worn or leaking brake master cylinder.
Brake Power Booster :
Damaged booster vacuum line/check valve, or diaphram .
Brake Proportioning Valve :
Improper installation/adjustment or damaged proportioning valve .

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there is a proportioning valve to the rear brakes that only allows them to operate fully under load or heavy braking. If it is faulty then full brakes will be applied to the rear which will allow them to lock up first.
Check pad/shoe condition and rear drum/shoe adjustment as well

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.

proportioning valve

possiable brake booster. if you hear a hissing sound and truck runs rough if not possiable master cylinder.

There is no way that your proportioning valve is going to affect your engine idle. However, a leaking master cylinder and the resulting damaged diaphram in the vacuum brake booster will. Remove the master cylinder and look for brake fluid in the booster assembly. If there is fluid in there, replac both the master cylinder and the booster.

if something goes wrong with abs the brake system it reverts back to standard system!! If you can pump brake pedal up and hold it and start engine and it drops thats normal.You say low pedal if you go 15mph on road and check where pedal is when brakes apply.Power brakes will push aLMost to floor while stopped but while moveing you`ll almost go to slide or will go into slide if pedal is pushed to floor.Get on some gravel at 10 mph and hit brakes to a complete stop get out and check to see if all 4 tires slid or 2 front or only 2 rear. this will get you closer to problem good luck.

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. 

Do you have a/the proportioning valve in your system? Pin needs to be pulled out for centering while bleeding.