Weird C10 Front Brake Problem - One Locking Brake

[JohnIL]

Hey fellas,
I need some help with the front brakes on a '77 C10. This is my son's daily driver. He's fine under controlled braking. But, if he's in traffic and he has to get on the brakes hard, the left front locks. of course, it's much worse on wet roads. My uneducated guess was that he either had a stuck caliper or collapsed hose on the right. In the name of caution, we replaced both calipers and both hoses. After bleeding the system, the problem is exactly the same.

The brake system is stock. Front discs, rear drums. Stock master cylinder and proportioning valve. Original steel hard lines.

Any ideas? What else would cause one disc brake to be MUCH stronger than the other?

John