Antilock Breaks on 2001 GMC Sierra 1500

The Right Front Brake locks occasionally. At first it seemed like a vapor lock or some type of heating problem. It only does this after I’ve driven the truck for ~30 min then a short stop then try and go again. After I use the brakes for the first time after the second stop the front right brake won’t release. If I let the truck sit for an hour or so the brake releases and no more problems – for awhile…
It appears that something is holding pressure on the break line. If I bleed the brake while the wheel is locked , the fluid shoots out and I can see and hear the caliper release. I have replaced the caliper thinking that it may be jamming internally, but with not change. If I play with the Antilock brakes by slamming on my breaks causing the pump to operate, it seem to solve the problem for awhile. I’ve also noticed that just before all of the trouble begins that the truck will pull left when I apply the brakes . kind of like the left front is catching. I’m thinking the Antilock Brake pump may need replacing? I don’t think it’s the Master cylinder; because, if it were wouldn’t it lock both front brakes? There is only two lines from the master cylinder to the ABS pump. I get no indicator of any malfunction in the ABS system.

Asked by for the 2001 GMC Sierra 1500
While the brake is locked, try breaking the lines free at the master cylinder, one at a time, see if the pressure releases. If it does, that is your problem. If that doesn't help, try the lines to the ABS Hydraulic Unit. I don't think it's the pump, you would hear it running or it would apply other wheel too? It's not the pump that holds pressure or releases pressure to the wheels, that is the modulator valve I believe.