Where is this proportioning valve and what does it look like. When I havent driven my car for a while my right front locks up the first time I hit the brakes. I have changed all calipers lines to the rear and master cylinder. I have all four drum brakes.
I would replace the brake hoses. Many times, caliper failure is due to faulty brake hoses. As pointed out, master cylinders are inexpensive, too. That's where I would start, anyway. Jim
Make sure the calipers/pads are free on the mount.I have seen many frozen pads/caliper mount bolts(sliders)frozen up causing brakes to overheat and lock up.Do the brakes stink when this happens???Chances are its a bad prop valve or hoses.Was the brake fluid in the M/C grungy looking when you serviced the fronts.If so that prop valve is rotted partially shut causing the pressure to the front wheels to not bleed off.Try this:Next time it happens,crack the line loose at the master cyl.If it spits fluid at you like its under pressure its the M/C if it doesnt.Crack a bleeder loose at the calipers.If it spits like its under pressure its the hose or the prop valve.Hope this helps you.The prop valve is down low at the rail,follow the lines from the M/C they will go right to it.
If you havent,I would recommend clearing all the old fluid out of the system once you slove this problem.Brake fluid is a water magnet,it sucks it out of the air.Main reason for hydraulic brake component failure.Hope all this help you out.Try and prove out the components before replacing them.The cost adds up fast when your chasing gremlins(I dont mean the AMC kind either)Good luck.
Sound like you got a bad brake line. Check the rubber line and check the hard lines for kinks or dents.