[wilkin250r]

Just to lend yet even more credibility to the brake line theory. I've personally never seen it with the rear brakes, but I have seen it with the fronts a few times.

The other possible issue is the brake fluid itself. I imagine you've bled the brakes, but have you REALLY bled the brakes? Enough that you've flushed all the old fluid out and replaced it with new fluid? The brake fluid will absorb moisture over time and lower the boiling point. It can get to the point that any heat in the system will cause the fluid to expand, which creates more pressure, which creates more friction, which creates more heat and thus creates even more pressure, friction, heat, ect. You can bleed the initial pressure off, but the cycle will repeat until you remove the moisure by completely flushing the brake system of all the old fluid.