Some of the more commonly-asked discussions on the Ford truck forums and groups revolves around the brake setups...specifically, proportioning valves...as well as several other brake-related components that are consistently misidentified as proportioning valves. Knowing what each of these different components actually are and how they function is the key to understanding how your truck's brake system works and how to properly troubleshoot it when problems arise, so let's discuss these in detail...

First of all, let's start out with a few facts:

1) All Ford F100s up through 1972 only had drum brakes front and rear. However, disc brakes were an available option on F250/350 starting in 1968. They became standard equipment on F100s in '73.
2) Vehicles with 4-wheel drum brakes don't need a proportioning valve, while vehicles with disc brakes and rear drums DO need a proportioning valve! (More on this later.)
3) Ford did not start installing a disc-brake proportioning valve in their trucks until 1973. Even the '68-'72 F250/350 with front disc brakes didn't have a proportioning valve. Their brake 'add-ons' include only a brake pressure differential valve and a stand-alone metering valve.

Starting in 1968, Dayton dual-piston caliper disc brakes were an available option on F250/350 models only (shown at right). This was a new technology for Ford and they were still trying to fine-tune them and match them to the completely-different operating characteristics of the rear drum brakes.

Therefore, the brake proportioning valve as we know it today hadn't even come onto the Ford truck scene yet. The brake line circuits only contained two 'add-on' components: the brake pressure differential valve and a stand-alone metering valve. Both of these two components are often improperly misidentified as a proportioning valve....and that misidentification tends to cause a lot of problems.

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So let's examine these two components and discuss what they actually are.

The little device mounted inside the left framerail, just below the driver's seat area, is a BRAKE PRESSURE DIFFERENTIAL VALVE. It is NOT a proportioning valve, nor is it a 'distribution block'! It's important to understand this...and what's it's actual function is. It has no proportioning features to it whatsoever and doesn't affect brake performance in any way. The stand-alone brake pressure differential valve is used on all '67-'72 trucks, 2WD and 4WD, regardless of whether the truck is equipped with front disc or front drum brakes.

1 - Rear
2 - Left-front
3 - Right-front
4 & 5 - Master cylinder

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This diagram from the Ford Truck Shop Manual isn't 100% accurate, as the factory piece actually used has 5 ports (plus the switch port) as opposed to the 4-port Mustang version showed here. However, the principle is the same.

Unlike a metering valve or a proportioning valve, a pressure differential valve doesn't have any preset pressure settings to delay or to reduce the hydraulic pressure being applied. It's just a spool suspended and balanced inside the valve body between equal pressures of the front and rear brake circuits. If the pressure in either circuit is unequal, the spool is pushed to the low pressure side, closing the switch contacts and illuminating the dash-mounted warning light...simple. It doesn't have any metering function for the front brake circuit and no proportioning function to the rear brake circuit like the all-in-one brake assemblies (see 'Combination Valve' below) installed during the '73-'79 trucks.

Once the problem that caused the warning light to illuminate has been corrected, the piston must be re-centered to turn off the warning light.

The warning light mounted on the lower-left of the dash, just above the parking brake assembly, is often  mistakenly associated with the parking brake, due to it's proximity. However,  as mentioned above, the dash-mounted brake warning light is only hooked up to the brake pressure differential valve and is NOT connected to or associated with the parking brake in any way!

On '68-'72 models, if the system is working properly as designed, the warning light will come on when the ignition switch is in the "START" position as a way to prove that the bulb and circuitry is functioning correctly. It shouldn't come on in any other key position. However, '67 has only one wire on the pressure differential switch and doesn't have the Prove Out circuit of the 2nd wire to make the warning light momentarily flash in the start/crank ignition switch position. The '67 pressure differential switch takes a 1/2" wrench/socket to remove/install. The '68-'72 pressure differential warning switch will require a 9/16" wrench socket to remove install, but the threads at the base of both switches are 3/8"-24, so they can be physically interchanged at the valve body.

The owner's manual gives a brief description of the warning light and it's function. Some truck owners misinterpret the 'brake warning light' label as meaning parking brake, but it's actually for the hydraulic portion of brakes. From the 1969 owner's manual:

One myth that often pops up during conversations is the misconception that if the spool in the pressure differential valve gets pushed off-center due to (for example) a brake line failure, it seals off the bad side, allowing you to still have brake pressure on the good side. This is incorrect! It is merely a warning system. The ends of the spool are turned down to allow brake fluid to pass when it is pushed to one side. If the spool was designed to cut off the brake fluid to the side where the pressure has dropped it would seal the fluid between the master cylinder and the distribution block. This would cause the brake pedal to be rock hard and you would have no brakes at all because the trapped fluid would not give. The dual master cylinder is the safety feature that allows pressure at the good end. There is nothing inside the pressure differential valve to replace when rebuilding except for two special o-rings and a crush washer.

Reseal kits are available for some pressure differential valves through www.MuscleCarResearch.com. These utilize the special square O-rings necessary to withstand the corrosive nature of brake fluid. Do not use Buna-N (Nitrile) o-rings in the brake system as it is not a compatible material. Butyl or EPDM are the only commonly available materials you should use. Others include Virgin Teflon, Styrene Butadiene and Aflas.

A rebuilding tutorial on PDF is also available HERE.

NOTE: The only difference between the drum/drum pressure differential valve used on cars vs. trucks is their mounting brackets. The valve body and internals/seals themselves are the same.

There were several different types of pressure differential valves used on the dentside-era ('73-'79) 2WD trucks. Primarily though, there were two that were most commonly used. These two are pictured here with the typical bumpside-era piece (left). Usually the cast iron Kelsey-Hayes brake valve (middle) was used in trucks 6,800 lbs or under and the brass Weatherhead valve (right) was used in trucks over 6,800 lbs.

The stand-alone disc brake metering valve (a.k.a. "hold-off" valve) was only used on '68-'72 F250/350 2WD trucks with the optional dual-piston caliper front disc brakes. It's located on the left/rear area of the main front frame crossmember and plumbed inline into the front brake circuit between the brake pressure differential valve and the front disc brake calipers.

The metering valve is just a hydraulic pressure delay device. It gives the rear brake circuit a little bit of a head start for the wheel cylinders to fill with fluid, overcome the tension of the brake shoe retracting springs and begin moving the shoes toward contact with the drums, before the front discs begin to apply.

I don't know the specific pressure rating of these particular metering valves but it's generally around 100 PSI (give or take). Before this point, the metering poppet valve is held closed by a coil spring, of a given wire diameter. Once the pressure in the brake lines builds to a sufficient level to overcome the tension and compress the poppet against the spring, the poppet is forced open and brake fluid and pressure are allowed to reach the brake calipers thus, applying the front brakes in conjunction with the rears.

1968-72 F250 2WD & F350
1973-78 F350
1973-79 F250 2WD w/single or dual piston caliper front disc brakes
1979 F350 2WD

In 1973, Ford integrated the pressure differential valve and the metering valve into a single valve body assembly on the disc/drum trucks. And for the first time in the trucks, a third function (proportioning) was also added to this integrated brake valve assembly, so it now had 3 separate brake functions:

1) a metering valve (for the front disc brakes)
2) a proportioning valve (for the rear drum brakes)
3) the familiar brake pressure differential valve (sensing in between the primary and secondary brake circuits)

People often mistakenly refer to this complete assembly as the proportioning valve...and while that's not entirely inaccurate, it's actually called a 'combo valve' because it includes three different components with completely different functions integrated into a single unit...it's more than just a proportioning valve.

Contrary to popular belief, the proportioning valve does not limit the pressure to the rear brakes. It reduces the rate of pressure rise to the rear brakes in proportion to the pressure being applied to the front brakes during a hard stop.

Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes. The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located.

If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.

So how does it work? Well, under normal braking use, the poppet of the proportioning valve sits dormant and does nothing. This means under normal braking, the pressure being applied to the front brakes is also the same pressure being applied to the rears.

In the event of a panic stop, the brakes' hydraulic system experiences a sudden spike/rise in line pressures and there's a sudden shift in vehicle weight from the rear to the front. The nose dives and places more downward force on the front tires. The upward lift of the rear unloads the weight over the rear tires.

This reduction in rear weight makes it very susceptible for the rear drums to lock up. The abnormally-higher line pressures to the rear causes the fluid to push on the poppet of the proportioning valve. This overcomes the tension of the coil spring of the poppet valve and pushes the valve over the fluid passage to create a partial blockage. This reduces the opening of the valve body fluid passage and reduces the rate of pressure rise being applied to the rear drums.

Factory proportioning circuits are engineered for the vehicle it's installed on. Different vehicle GVWs will dictate the spring rating on the proportioning poppet valve and when it will begin to operate (knee-point of the pressure curve in proportion to the pressure being applied to the front brakes).

Not all OEM proportioning valves come with the same spring/pressure rating so they will not all become active at the same pressure application point in the brake system. So that's the reason Ford listed the proportioning valves for the light 6200# GVW 4x2 F250 dentside pickups with the F100/150s...and the heavier 6900#, 7500# and 8100# GVW 4x2 F250s with the 4x2 F350s.

 

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If you look at the front end of the combo valve, there's a brass pin just in front of a rubber boot. This is the metering valve bleeder and is something you don't want to forget about when attempting to bleed your front disc brakes. A special tool pulls outward on the pin to allow the brake fluid to reach the caliper assemblies. This operation is particularly important if pressure bleeding is being used.

In a pinch,  you can use a small pair of ViseGrips to hold the pin out for bleeding purposes, but it's recommended to either find the proper tool or build yourself one from a piece of sheetmetal.

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