First time owner working on the brake system and found this regarding the proportioning valve effects:
1) S/N 9193
2) 16,900 total miles. Inactive for last 18 years.
3) OEM Girling calipers with Textar pads. 1 piston aft, 4 piston front.Solid rotors.
4) Previous owner never worked on the brake system so I suspect it is all factory original.
I am new to Panteras and no expert on auto brake systems. I noticed the wear pattern on the pads and my first thought was that the front brakes are not working. The front rotors show absolutely no wear but I have not measured them.
I was reading the Dec. 2011 Sticky 5 post On the PV (proportioning valve)and thought this was a good example of it's operation.
I would think that if you were trying to balance a unbalanced (4 piston fnt. to 1 piston rear) brake system by use of the PV that you would have made it adjustable.
I have yet to drive the car but from what I'm seeing the brakes must have been poor at best.
As is I would think the system would benefit from removal of the PV. You need the fnt. brakes working.
QUESTION, does anyone have a diagram or discription of how the PV works ?
I would like to think on it awhile without taking it apart. Also not done reading the post or others on the subject.
Trying to keep the car original but stopping is a priority. Not that we'll ever see second or third gear here in LA traffic.
Last thought, as is the unadjustable PV seems to me like a patch for a poorly designed / way unbalanced brake system. But I'm a retired light aircraft mechanic, not a auto brake expert. From what I've seen on other cars the front brakes are most always bigger than the rear. And that's for a reason so why the PV ?
Original Post
The factory proportioning valve is non-adjustable, being set by a large spring inside to automatically reduce the max hydraulic force delivered to the FRONT calipers from the master cylinder. The small rear calipers receive 100% of the max possible hydraulic force. Since about 65% of the possible max braking is done by the fronts in a Pantera, this setup is WRONG, giving away significant brake force from the 1970s racing-inspired front calipers.

Worse, the valve setup assumes the tires on the car are OEM 185-70 x 15"(frt) and 205-70 x 15" rear (optional 215-70 x 15"), on 7" and 8" widths, in a fairly hard rubber compound no longer used. So if you are NOT using that size and type of tire, the valve has the wrong spring besides being hooked up backwards. This is why most owners that actually drive their cars discard the stock valve and replumb the whole brake system as the designer Dallara likely intended. A manual adjust proportioning valve can be substituted to balance braking force front-to-rear once the system is plumbed properly.

All this, along with the swaybar sizes and a few other things were done (probably by Ford Engineering) to make the mid-engined Pantera behave like a front-engine Ford Galaxie for the average, untrained U.S. passenger car owner.
Below is an excerpt from our instructions when installing one of our brake kits. I think it will answer most questions:

Most cars have proportioning valves on the rear brakes to keep them from locking prematurely before the fronts. Not on our Pantera’s. DeTomaso thought it was a good idea to put a proportioning valve on the front circuit of the brake system. The reason why they did this was to try and balance the poorly designed rear brake system when it comes to brake system balance. Basically what they did was to detune the front brakes so they matched the rears a little better. The fronts were great brakes but then they detuned them because the backs were so poor. Quite a few Pantera owners have removed the proportioning valve from the stock brake system for a free upgrade to the braking performance of the front brakes (see important note below). This modification coupled with a rear brake system upgrade provides a significant upgrade for a relatively small price. The proportioning valve should be removed from your car once this kit is installed. It is highly likely that the rears would lock up before the fronts with the proportioning valve still in place. This would be a very dangerous condition that needs to be avoided.

IMPORTANT NOTE: If you purchased just front calipers and are still using the stock rears, DO NOT REMOVE the stock proportioning valve at this time. Remove it, sometime in the future, when the rear calipers get upgraded. The reason for this is because with the higher pressure going to the front Calipers (if the P-Valve was removed), the front Calipers will be stopping the car way before the rears get enough brake pressure to assist in the stopping of the car. In fact, removing the proportioning valve without upgrading the rears is basically disabling the rear brakes. This is again, because they never will get enough pressure to actuate. To get enough brake pressure for the rears to actuate will cause the front to lock up. This is the exact reason DeTomaso installed the P-Valve in the first place. A better solution would have been to improve the poorly designed rear instead of lowering the front brake pressure. What they affectively did was to increase brake pressure to the rears by lowering pressure to the front, thus achieving better balance.

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