Brake pressure differential valve. what ever you want to call it. The brake light is on.

Does this thing actually slide over and cut off the rear brakes from getting any fluid at all? I put new wheel cylinders in and im trying to bleed the rear brakes but im not getting anything coming through them. No burst of air or anything. Ive tried leaving the bleeder screw loose and letting the gravity feed fluid to them but its not draining any out of the master cylinder.

So could the piston be slid over and cutting them off making it impossible to get fluid to them?

Ive tried resetting it the way the haynes manual says but it doesnt go off.

Maybe im wrong but im thinking that the piston is slid over and is blocking the rear line, keeping me from being able to bleed the lines?

 

I really hate those things, and wish I had an answer for ya, I feel your pain and just don't want to leave your thread unanswerd. Hopefully someone will chim in with a miracle solution.

 

In the 70's various manufacturers tired to come up with an answer to a government mandate. Their challenge was to make a warning light come on telling you that your brakes had failed. The solution? A piston that slid back and forth in a bore responding to a loss of pressure in the front or rear braking "circuit".

Back in the day ... because the light didn't come on until you stepped on the brakes ... the joke about these systems was that they didn't tell you anything that you didn't already know!!!


I am familiar with the devices used in foreign makes ... I have to admit that I have only a limited working knowledge of American systems ... I promise I will do my homework and come up with the op of the Ford system.


BUT ... all of the systems I am familiar with all act about the same ... a loss of system pressure in one circuit ... allows pressure from the working circuit to move a piston ... and allow a pin in a switch to rise (or fall depending on design) closing a circuit to ground and making a light come on.


In most of my experience ... these systems do NOT allow a piston to shift so far as to close off a circuit ... but anything is possible.


BEFORE GOING ANY FURTHER ... MAKE CERTAIN THAT THE REAR BRAKES ARE ADJUSTED PROPERLY ... AND BLEED THE MASTER CYLINDER.

Bleeding the master cylinder involves 2 people ... the person under the hood ... removes both brake lines from the master ... takes the cap of the master ... and, keeping the brake fluid level above the suction ports ... he covers the holes the brake lines fitted into with his fingers.

Then he tells his partner ... seated behind the steering wheel ... to press down AND HOLD PRESSURE on the brake pedal.

The person under the hood will lift his fingers slightly from the holes. This should allow a brake fluid/air mixture out of the master cylinder. He will then tighten his fingers on the holes ... and tell his assistant to "LIFT SLOWLY". This will cause the pistons in the master cylinder to suck in fluid from the reservoir.

Repeat the procedure until you get a good, strong jet of fluid from the holes in the master.

IF NO PRESSURE BUILDS UP DURING THIS PROCEDURE ... YOU HAVE A BAD MASTER CYLINDER.


If you get good pressure from both holes go ahead and install the brake lines back into the master cylinder.



The "Brake pressure differential valve. what ever you want to call it" device is located about 12 inches (+/-) below the master cylinder. You will identify it by a number of brake lines ... and a couple of wires going to it.

There a chance that the piston MAY have traveled to far in one direction or the other...and the pin in the switch is holding the piston to one side ... I remember a million years ago seeing this happen on an old MG or Triumph.

I would unhook the wires ... and remove the switch from the valve ... make sure the pin in the sensor/switch moves freely ... then re-bleed the system using the "coke bottle and hose" method I discussed in post 5 (I think) of this thread:

https://www.ford-trucks.com/forums/8...no-brakes.html

After following this procedure, the pressure SHOULD return to the system AS LONG AS:

a. The brakes are adjusted properly
b. you don't have a loss of pressure somewhere else
c. The master cylinder is OK

If you get the system bled properly ... replace the switch.


This SHOULD work ... you might have to install the switch back into the valve before bleeding ... but I'm not certain of that.


BTW...

The "Coke bottle and hose" bleeding system has NEVER failed me ... when other methods had.

You can also bleed out a master cylinder quite well using this method ... but it requires you have a couple of lines and fittings that will screw into the master.

Let us know how you make out.

I PROMISE to do my homework on this valve.

 

I KNOW John Galt. I hope others get to know him as well.

 

Ive got pressure with the pedal but im just not getting any fluid to the rear brakes. Ive tried to syphon it through but didnt work. this thing is driving me crazy.

 

Not quite right, they are all designed to slide the piston to the side that is leaking and to block it off. This is to prevent the master cylinder draining empty and leaving you with absolutely no brakes. Of course, if your leak is somewhere between your master cylinder and this valve then you're hosed.

My daily driver has this device cast into the master cylinder itself, probably a later regulation.

I would suggest to try and bleed the 'empty' line up to the valve then absolutely stomp the pedal. This should theoretically blast the piston back into position. Even crack the 'good' line to emulate a leak and stomp the pedal will try and push the poston the other way. Might get stuck there though...

Best off, replace it. The proportioning part of it is probably stuffed as well.

 

I say just get rid of the darn thing, you don't need it for the brakes to work right, and it only causes trouble. Replace it with a "T" and a juction.

It's not really a proportionaing valve anyway, it's just a juction block with that stupid slider valve and warning light switch.

 

try pushing hard on the pedal and opening a front bleeder quickly. This will usually let the piston recenter. Watch for the brake light to go off.then try vacuum or gravity bleeding the rears. If you have to use the pedal to get fluid to the rear push REAL slow on the pedal with a bleeder open thrn close the bleeder and release the pedal

 

XzXPatrickXzX, have you bled the MC per Oddshot's instruction? The last time I was in this situation it was due to air in the MC; a good bench bleed did the trick (and it was the rears too).

 

Sorry Stang ... that isn't correct.

The Tandem Braking system came into common use approximately 1967. This brake system is divided AT THE MASTER CYLINDER into 2 separate circuits.

On the Ford Pickup and most non-front wheel drive vehicles, this system is divided into a a front circuit and a rear circuit. On some makes, especially front wheel drive types, the system is divided diagonally, left front/right rear ... and right front/left rear ... but the system is still divided into 2 circuits ... separate, but equal.

In the Ford Pickup tandem braking system we have 2 separate reservoirs and pump chambers in the master to prevent total fluid loss in case of a leak in one circuit or the other. Some modern vehicles have gone to a single fluid reservoir ... but they still have a split master.

AT NO TIME AND IN NO PLACE does the fluid OR pressure from one circuit get into the other.



On another note ... DO NOT REMOVE THE PRESSURE DIFFERENTIAL VALVE (aka erroneously, the "proportioning valve") from your Ford Pickup.

The warning system build into it might be a joke ... but the work of this valve is NOT!

This valve is VERY IMPORTANT to a "nose heavy ... tail light" vehicle like a pickup truck. During braking, this valve LIMITS the hydraulic pressure to the rear brakes to a predetermined level ... while pressure to the front brakes is allowed to continue to build.

The difference between the PRESSURE DIFFERENTIAL VALVE and a PROPORTIONING VALVE ... The proportioning valve allows brake fluid pressure to the rear brakes continue to grow, albeit at a very reduced rate. The Pressure differential valve will NOT allow pressure to go past a specific, preset value.

Now I haven't taken one from my Ford and had it apart on the bench ... but all my manuals indicate GENERICALLY this valve CANNOT completely shut off fluid to the rear brakes.

If it could ... you would NEVER be able to bleed out a brake system. Also ... under very severe braking there is a risk that this valve could
HOLD system pressure ... meaning the the rears COULD stay locked on ... when the pedal was released. Not a good idea.

Also ... the cutaway drawings I can find of the Ford Pressure Differential Valve show me that it will limit the pressure ... but not cut it off entirely ... it just ain't machined that way. But like I said ... I ain't had one apart.

Yet.

 

With any automotive repair that become a "problem" ... I have always tried to go back to the basics.

This job is no different. This is a braking system that was designed in the 1960's. How hard can it be?

Hit EVERY step and you should have your answer.

1. Make sure that the real brakes are properly adjusted and working properly MECHANICALLY.

2. Make certain that you do NOT have a CRUSHED OR BROKEN brake line anywhere in the system.

3. Bleed out the master cylinder. Even if you think you have good pressure. MAKE CERTAIN the master is good!

4. Bleed out the brakes using the "coke bottle" system I described.

Before you begin ... you *might* want to loosen or remove the switch from the Pressure Limiting Valve ... and stand on the brake pedal good and hard a couple of times ... just to make certain the switch hasn't held anything open/closed.

If tapping something makes you feel good ... go ahead and TAP on the body of the master cylinder ... and the body of the Pressure Limiting Valve ... it can't hurt.

Let us know how you make out.

 

this might help.https://www.ford-trucks.com/forums/8...rtioning+valve

 

sorry guys for not replying sooner, i have been at the junkyard and work all day.

I picked up a spare proportioning valve while i was there. I was going to take it apart and see how it works but thanks for the link showing it already apart 86bigred.

I have to work tommorow but ill bleed the MC by odd shots method tommorow and try to get some fluid to the rears. if that doesnt work ill try what bashby said with the bleeders. I hope to god one of them works.

BUT before i do this how do i adjust the rear brakes properly? and how do i know when they are adjusted? i know to turn the adjuster the on the bottom of the inner drum, im guessing clockwise to tighten and counterclockwise to loosen?

Also, currently my parking brake cables are disconnected and wired to the frame. Even tho i plan on getting new cables in the future, do I have to have them hooked up to be able to adjust the brakes properly?

I really appreciate your guys help tho. Im no mechanic if you guys cant tell but im learning as i go. I know im probably frustrating you guys asking the same questions over and over but ive really never dealt with the whole brake system before and i have no one to help me except a guy that thinks he knows everything, or someone that is too worried about messing around instead of paying attention and actually helping.

Off Topic: I picked up an extra proportioning valve, a power steering pump (2 actually but one looks fine but the bracket is broken and another that the cap was missing from and probably shot but it had an unbroken bracket that i switched over to the good one), a glove box since mine was made out of cardboard like on the back of an entertainment center to be able to fit an after market air conditioner in behind of it (which ill be taking out because it doesn't work), a washer fluid reservoir with a nice motorcraft sticker on it since i busted a hole in the original one. And last but not least a the whole dash cluster with an oil pressure gauge, an ammeter gauge. Ive read on here that it will be a hard conversion from idiot lights but i got the plug and about a foot an a half of the wiring. I also got the speedometer cable from it too since mine doesnt work. SO heres another big question which may be stupid but is the speedometer cable and gear tranz specific? im not sure what kind tranz was in it because the frame was on rims and weeds grown up underneath it. my truck has a C4 not sure if it has cruise control or not but the PO changed it from a stick to an auto C4. There are cruise control buttons on the steering wheel (which was also changed) but i have never tried them..

Also while im thiking about it. The linkage rod goes down and connects to a bracket to changed the gears. There is another bracket im pretty sure below it not hooked up to anything. no cable or nothing. I have been told that it is the passing gear and should connect to to throttle cable at the carb. Is that right?

Once again thank you guys a whole lot. I learn something every time i read on this forum. You guys are the best.

 

You might get a better response to your non-brake questions by putting them in a new thread.

Adjusting Brakes:

1. back off the adjustment for the emergency brake. This will allow you to properly adjust the service brake without the extra drag of a over tight emergency brake.

2. I've always adjusted Drum Brakes by tightening the adjuster until I can't spin the tire ... and then backing off on the adjuster ... about 8 teeth ... until the wheel is free. What ever amount of teeth I back off on one side of the truck ... I do the same to the other.

AS LONG AS THE ADJUSTERS HAVE BEEN INSTALLED ON THE PROPER SIDE ... pushing DOWN on your brake adjusting tool when you've engaged the teeth on the adjuster ... will turn the brake adjuster UPWARD ... this expands the adjuster and will tighten the shoes against the drum. Spin the tire while adjusting ... when you can no longer spin the tire ... stop tightening!


The problem with adjusting drum brake ... is that pesty self adjuster. When you've got the brakes adjusted so you cannot spin the tire ... reach through the adjusting hole with a long, thin screwdriver ... aim the screwdriver over the top of the adjuster, near the toothed section of the adjuster ... you are trying to locate a flat bit of steel ... the self adjuster ... you will want to push this steel towards the outside of the wheel away from the adjuster. The self adjuster will feel as if its "spring loaded" and will have a little "push back".

Holding the self adjuster away from the teeth of the brake adjuster ... lifting UP on your adjusting tool ... will move the teeth of the adjuster move DOWNWARD ... this will retract the adjuster and loosen the brake shoes from the brake drum.

As I said ... I usually loosen 8 teeth ... or until I can spin the wheel freely.

Do the same to both wheels.

3. Re-adjust the Emergency Brake. Tighten the adjustment nut on the cable until you begin to put a drag on the rear wheels ... then back off the adjustment until the wheels have no drag.

 

Ford Head here! NAPA offers a one man bleeder kit for master cylinders for about $6.00 depending on where you live.