2000 Silverado ABS EHCU problem? front brakes locking up randomly HELP

Help, 2000 silverado 2500, front brakes keep locking up after driving for a while, pedal gets hard, brakes apply, stops my truck, just burnt up a set of pads, pull over let sit, brakes will be ok again till it
sits again, sometimes its ok for the day, changed calipers-pads-bled the brakes 3 times with synthetic, fluid does not want to go back up toward the cylinder when this happens if I jam on the brakes a bunch of times sometimes it frees up. typical scenario, leave my house to go to work, after 10 minutes on the highway without touching the brakes, the front end starts to shake, the truck slows down, the pedal gets hard as a rock, right up at the top, there is no leaks, changed the drivers front brake hose, changed calipers but that didnt help, its in the hydraulic unit or master cylinder.
ABS doesnt seem to work when this is happening, I can push hard on the pedal and lock the front wheels when this self applying problem is happening, no prolblems with the rear brakes at all.
This all happened after I put new pads in about 3 weeks ago, they had 60,000 on the old pads with no brake problems whatsoever, the only thing I can think of is I compressed the calipers with a C-clamp and pushed the fluid back into the master cylinder, which I have done a million times before, I was told that this is fine for the GM EHCU system.
Did I mess up the EHCU or some kind of check valve.
I have power steering pump assisted power brakes also.
The EBCM is not running and the ABS does work when this problem is not happening. I drove around in the snow for 1/2 hour yesterday and made the ABS works its ass off trying to purge the EHCU, and it was fine all day, when I went home from work that night the front brakes started locking up again on both sides, I mean smoking pads.
HELP..
Billymac in Boston
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Get it to act up, raise one or both front wheels off the ground. Crack open the front brake line at the master cylinder, see if front is now free. If yes, the master cylinder is not releasing. If no, move down to the fitting(s) for the next component (HCU), keep going (hose then caliper) until you isolate which component isn't releasing the front brakes. Start at the top and work your way down.
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