Yes there is something wrong with doing that; it's a serious safety issue. The factory valve isolates the front and rear braking circuits so that if you have a failure on one end (front or back), you'll still have brakes on the other end. When it senses a pressure differential, it slides a piston toward the "bad" side to shut it off and maintain pressure on the "good" side.

If you eliminate this mechanism, a leak on any one of the four wheels will kill brakes on the entire truck. That's because no pressure will be applied to the "good" side; all braking force will be diverted to the side with an opening. This means crashing into a tree.

Summary: do not do this. Even if you don't value your own safety, you owe it to the others on the road to value theirs.

 

I agree.

There are lawyers advertising on TV how they love to go after such people, this would be in addition to the manslaughter charges the DA is gonna hit ya with in the event of a deadly accident.

Don't expect the insurance company to cover you, either. Your truck came from the factory with certain capabilities as defined by the NHTSA (link below). If you defeat those functionalities, you alone bear the risk.

Operating a deadly weapon (motor vehicle) on the public streets and highways requires a lot of responsibility (this is why you are tested and licensed) please treat it seriously.

Sorry for the soap box, but once you've been in near-death accident, your attitudes change.

Federal Motor Vehicle Safety Standards and Regulations

 

Liability considerations aside. The proportioning valve DOES NOT act like a valve to shut off fluid flow.
The valve serves a couple of functions. It acts as a proportioning valve to regulate pressure front and rear to keep the rear brakes from locking up because drum brakes require less pressure to operate than disc brakes.
The second function is to signal brake failure. The way it does this is there is a small spool valve that is between the front and rear brake circuits. This valve is a small shaft that has a groove in the middle, the switch that controls the brake failure light has a pin that sits in this groove. When the valve is centered and there is equal pressure on both sides, the switch's pin is extended and the switch is "off".
If there is a failure in either the front or rear brakes, when you hit the brakes, the high pressure from the side that is still functioning will push the valve towards the low pressure side. This pushes the groove away from the shaft effectively pushing the pin in and turning the switch "on".
This function is there strictly to turn the warning light on or off, nothing more.
If a dual master cylinder is used, the front and rear brake systems are effectively isolated from each other and a failure in one system will not affect the other system.

 

I revisited my shop manual, and I am indeed wrong. The fail-safe function I was thinking of is actually implemented as a dual-piston assembly in the master cylinder itself and not in the valve. Sorry for the confusion and thank you Mike for the correction. Note, however, that the picture above is for a disc/drum setup. I assume the drum/drum setup in question would use the same mechanism for the warning switch?

That being said, you would still be bypassing your truck's mechanism to warn you of a braking failure and this could be used against you if a lawyer goes after you. In that case I would say Chris's point still stands. The braking system is one place where you shouldn't take any shortcuts, regardless of how harmless they seem.

 

fmc, you're correct. The valve pictured is for a disc/drum combo. The drum/drum isn't actually a "proportioning" valve but rather just a pressure differential sensor. It does use the same style valve to trigger the system failure warning light though.
Here' a drum/drum system valve.

 

Shotgun2,
Thanks for the reply, why isnt it really recomended? Just because you lose the low fluid warning feature? Is there any other adverse effects to plumbing the brakes without the valve on a drum/drum system?

Thanks,
Matt

 

I understand that the truck came equipped with the low fluid sensor switch, I have a dual reservoir MC, so front and rear brakes are isolated from each other, and it would be in ones best interest to keep the low fluid light switch functioning, however, with the prop valve/distribution block for the drum/drum setup obsolete, what other options are there, does anyone make an aftermarket low fluid light switch that can be plumbed in without using the original pv/db? Thanks for all the replies here guys, I'm a rookie here just trying to keep an old girl on the road.
Matt

 

I have driven one of these trucks that I baught (79 f350 4x4) that the brake line to the rear brakes was ruptured. No brake light ever came on and the pedal was spongy on the way home. It drained the fluid from small cavity in the MC and none from the large cavity. The small one goes to the rear brakes and the large one goes to the fronts. In the event that a proportioning valve was doing this, then it would have shut off the fluid to the rear circuit, correct? It didn't. It functioned perfectly later after the problem was correcred, so i have a hard time seeing as it was nor in operational order.

Moral is the brakes are seperated in the mastercylinder res. not at the PV. Thanks. My question is now that I have a truck that the PV is acting up, is there a way to test or rebuild them? I have new master cylinder, slave cylinders, calipers, pads, shoes, and my brakes feel about 85%. I did some more bleeding last night and with the brake pedal down, I broke the line to the front brakes at the PV. After that I didn't recenter it but drove it around in the yard and the back brakes grabbed a few times, then the brakes awesome for a while, so i took it out on the road and had good brakes. After about 5 stops, my brakes got less and less until they were same as before.

I shut the truck off, parked it and recentered the PV. Brakes good the trip home. Haven't drove it since. Any suggestions?

 

Can I ask what you mean by recentering the valve? I have done the whole front disc axle swap on my truck. Put the correct PV on it and changed to a disc master and booster from drum. My brakes are good, but feel they should be better. Have the same problem as f250 warhorse brakes are good for a little bit after bleeding but then the pedal gets soft, what am I doing wrong? Just saw that the valve on the PV needs held in, never did that before will have to try it when I get the truck back out.

 

It says in the directions to turn the key to on, and press the brake pedal as far as it will go to in one steady motion. This is supposed to recenter the valve in the housing, I have a light when cranked to start, but I tried to unplug the switch and hot wire to see if it would light the bulb, no luck I think I have a wiring issue somewhere as well, but mine will do just that, if you bleed or something, it feels fine then goes right to he!! after that. I think I need new rubber hoses as well as some PV work. I might try the hoses first.

 

You should be able to disconnect the wire's connector from the valve (I think it's white/purple but don't remember for certain) and ground it, that should make the dash light come on (with the key on).

 

I thought of that but I didn't do it. Mine has 2 wires, a purple and a white and purple

 

The electrical switch in the valve merely grounds that (those) wires when it feels the need; if all is well with the electrical connections and wiring, the dash light will come on.

Much of Ford's circuit wiring in this era works by completing a ground connection, not by supplying "positive" current.

 

Wel the light isn't my issue anyways, I would like to know why I have 100% brakes after bleeding or recentering and shortly thereafter they diminis. All is factory equipment, The only change was a D60 front, but that should be interchangeable on a 78 f250 4x4. I am thinking that the proportioning valve has some gunk built up and is hanging or sticking or something, anyway to try to correct this?

 

C8AZ-2B264-A .. Plastic Low Brake Fluid Warning Lamp Switch / Available from Ford/autoparts stores..

Applications: 1968/79 F100/350 & Bronco / 1969/79 Econoline / 1968/69 FoMoCo Passenger Cars.

The little charmer is notorious for developing cracks within its threads causing brake fluid to seep out.

When this occurres, the low brake fluid warning lamp on the dash comes on...and stays on.