[Evans, Marv]

This may not be it, but it sounds like the classic case of the space between the brake pedal rod and the master cylinder not being adjusted correctly. When the pedal rod & piston contact on the master cylinder's piston are too close, vibration of the pedal will cause the rod to barely vibrate against the contact face of the master cylinder piston. This causes pressure to be built up in the brake system which causes the brakes to bind up after a short time. I forget what the amount of free space should be (3mm comes to mind, but it's been so long, I'm not at all sure), so check out the free play from when the pedal starts being actuated and when it makes contact with the master cylinder piston. If I remember correctly there is a double nut on the rod to set the adjustment.