I seem to have a problem with the front brakes on my 78 F150. The disc brakes on the front seem to be way too tight. If you jack up a wheel it takes quite a bit of effort to roll a wheel.

I have replaced both front rotors, calipers and brake pads and it doesn't seem to help.

Is it possible that the proportioning valve may be sticking and holding pressure on the front brakes? Is there a check valve or something that I don't know about?

When I first got the truck I noticed that there had been water in the brake system. When I rebuilt the back wheel cylinders they were corroded from water contamination.

Is it possible that the proportioning valve may be corroded too?