Toyota Repair: front brakes locking up, wheel bearings, bleeder valves

Question
1985 toyota pickup, 4 speed, carb. 22r, 130k

1. front brakes, both, get hot and lock up.
2.  original problem was a wiped inner bearing, drivers side.  all bearings replaced on both front wheels.  brakes were fine before bearing repair, but lock up after about 4 miles.  
3.  replaced both front calipers, both lock up even quicker.  (pads were fine, did not replace)
4.  brakes remained locked up even after they cooled off for a day...had to 'burp' the bleeder valves...about a teaspoon of fluid shot out and then truck ran fine up to the local repair shop.
5.  repair shop replaced both front brake hoses, verifed and re-adjusted wheel bearings, excercised calipers and bled brakes.  Truck drove fine until two days and 20 miles later when it locked up hard again.
5.  help  

Answer
There may be a problem with the master cylinder, have the master cylinder rod in the vacuum boster checked for proper clearance, the master cylinder has to be removed and there is a special tool to measure the clearance between the master cylinder pushrod and the master cylinder piston, if this is not the problem replace the master cylinder.