Check the rod from brake booster to master cylinder. It needs to have some free play. Its not much. but if there is none or it's too long, the pistons in the master cylinder never return enough to open the return ports which holds pressure on the calipers. Since it stops straight, you can rule out one side or the other, follow the lines back to where they connect.
A simple test is to drive 4 or five miles till you know they are hot, lift it up and spin the wheels to verify they are tight, then open the bleeder and see if they loosen up. If it does, you have pushrod issues or master cylinder issues. If the do not loosen up, then the slides are the issue. If it is the in the master cylinder, both front tires should loosen up at the same time.
Another problem could be the master cylinder is plumbed wrong. It could be applying the rears first as most due, but since the lines are backwards, the fronts are always working the hardest. Be sure the rear shoes are adjusted up to the drums and you have a slight drag. Have a helper slowly apply the brakes as you rotate the tires, run back and forth from front to rear and see which one applies first ( a third helper would be handy ). I plumbed mine backwards once, it makes for some strange stopping.
