The Pantera Place
"Your de Tomaso Connection"

Pantera Brake Proportioning Valve

By Bill Taylor

 

All of the technical information and product information posted on this website is offered as general information and is not recommended, endorsed, guaranteed or presented as professional technical information.  It is recommended that you seek professional help for the repair or maintenance of your car.  Improper self-maintenance or repair of your car can result in serious injury and or death.  Use of this information is done at your own risk.   

The brake portioning valve from a Pantera is shown in Fig. 1. The valve has a single inlet port which receives pressure from the front brake port on the brake master cylinder and two outlet ports which deliver pressure to the left and right front wheel brake calipers. The brake proportioning valve functions to reduce the pressure applied to the brake calipers from that which is delivered by the master cylinder. In normal application, a brake proportioning valve is used to reduce the pressure applied to the rear wheel brakes. This reduction is needed to prevent the rear wheels locking up under hard braking when the vehicle weight is transferred to the front wheels. Because of the mid-engine configuration and the original skinny bias-ply tires the Pantera differs from normal practice in that the proportioning valve is used to reduce the pressure to the front brake calipers.

 Taylor Brakes Fig 1.JPG (39316 bytes)

Fig. 1

The test setup shown in Fig. 2 was used to measure the transfer function of the Pantera proportioning valve. This fixture consists of a master cylinder to create the inlet pressure to the proportioning valve, a threaded rod assembly to move the piston in the master cylinder, three junction blocks to accommodate pressure gauges, and the proportioning valve.

The master cylinder used in the test setup has a 15/16 inch piston. The piston area is approximately 0.69 sq. in. To generate 2,000 psi, the activating screw applied approximately 1,380 pounds of pressure to the piston. It should be clear that substantial strength is required in brake pedal, brake vacuum booster, and brake system mounting structures.  

Taylor Brakes Fig 2.JPG (30611 bytes)

Fig. 2

The test setup applied pressure to the proportioning valve in 100 psi increments from 0 to 2,000 psi. The two outlet ports of the proportioning valve had identical pressures. The Inlet Port and Outlet Port relationship is shown in Fig. 3.

 Taylor Brakes Fig 3.JPG (30133 bytes)

Fig. 3

The pressure at the outlet ports of a Pantera proportioning valve is reduced approximately 20% from the pressure at the inlet port.

I disassembled the test valve. The internals of the valve are shown in Fig. 4. And the components of the valve are shown in Fig. 5.

Taylor Brakes Fig 4.JPG (54108 bytes)

Fig. 4

 Taylor Brakes Fig 5.JPG (19016 bytes)

Fig. 5

If you do not desire the OEM reduction in pressure to the front brakes but you want to keep a stock appearance, the action of the Pantera proportioning valve can be disabled as follows:

     1)     Remove the valve from the car.

2)     Remove the 36mm sealing nut. (Mine required a stout vise and a 3/4 inch breaker bar.)

3)     Using a drift and a hammer, drive the internals of the valve out and discard them.

4)     Drill for, tap, and install a pipe plug in the valve opening opposite the sealing nut.

5)     Reinstall the sealing nut and copper gasket.

6)     Fill the proportioning valve with brake fluid and reinstall the valve on the car.

7)     Bleed the air from the proportioning valve and the front brakes.

8)     Install the rubber cap on the proportioning valve so the pipe plug will be covered.

 CAUTION 1: Brake fluid will damage your paint.

CAUTION 2: In doing the above modification, you are altering the brake performance balance which was set when the car was designed. I am not advocating making this modification and am not responsible for the consequences of anyone making such a modification.

1971 Valve Images 

By Mike Dailey

Proportioning valve 1.JPG (98075 bytes) Proportioning valve 2.JPG (103484 bytes) 

Proportioning valve 3.JPG (99914 bytes) Proportioning valve 4.JPG (104857 bytes)

Proportioning valve 5.JPG (163110 bytes) Proportioning valve 6.JPG (161921 bytes) Proportioning valve 7.JPG (94706 bytes) Proportioning valve 8.JPG (100110 bytes)

The rubber boot/seal was missing on bottom of this valve and the bottom part of the piston shaft was corroded and seized to the housing.  Because of the corrosion on the shaft this valve is no longer usable.   Malfunction of this valve can cause some interesting and hard to diagnose braking problems.  If you are having braking problems, including weak braking or odd pedal action than normally would be diagnosed as a bypassing master cylinder don't overlook this valve as a root cause.  

There are some in the de Tomaso Pantera community that believe that if you are running modern wide sticky 245 tires on the front and 335s on the back, that the braking can be significantly improved by replacing the valve with a T fitting.  Modifications to braking systems and or elimination of this valve are NOT recommend by Pantera Place.  Modifications to any of braking system components can cause dangerous and or lethal braking problems.                              

If a person was going to install a T fitting it might look something like this.  

Front Brake T.JPG (148643 bytes) 

The T is located to the left of the original valve position so the original lines could be used.  The stock Pantera brake lines cannot be double flared because the flare will split.  The extra line length was kept to reduce any strain on the T fitting and because it takes a number of flare attempts before a usable flare can be made.         

 

 

 

 

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