Pull up a chair- brakes are the second-most modified system on a Pantera (after tire sizes). First, if they haven't been changed, the 29-yr-old rubber brake hoses should be instantly replaced, either by new OEMs, or by Aeroquip braided-stainless. Second, the OEM non-adjustable proportioning valve should be discarded; it routes full pressure to the rear brakes and drops pressure to the fronts- where most braking is done. It is designed for bias-ply 185-70x 15 fronts and 205-70x15 rears. Either use a manual proportioning valve, or no valve at all. Be careful in rerouting this plumbing- the stock setup is a dual circuit so the frt & rears are separate; combining them will remove a safety feature. Third, there is a 'brake warning light shuttle valve' under the power brake unit. This ass'y has a brass slider inside that moves with brake fluid flow and if the pads are worn too far, turns on the brake warning light on the dash. The slider sometimes gets stuck at one end of its travel and effectively cuts off either the front or rear brakes! This can happen during bleeding, too. To fix this, either use an air compressor to recenter the slider, or disassemble the unit and clean the brass slider. Finally, you may want to look at the brake pads. If the pads were not broken in properly, they may have glazed, which will form a gas bearing between the pads & rotor. If so, scotch-brite the pad surfaces and rotors.