Centering Pressure Differential Valve

[foothilltom 33]

Howdy gents, I'm trying to turn off the bright red "BRAKE SYSTEM" idiot light in my coupe. I'm 99.5% certain it's a result of something called the Pressure Differential Valve...the gizmo that somehow indicates if your front or rear brake systems are not holding up their end.

 

I've bled the brakes (rear first, front disk last) and I've read that this makes the little ball valve shift to one side.

 

So my question:

 

How does one re-center that puppy? The shop manual is pretty vague but it sounds like it wants me to loosen one of the brake lines that goes into the sides of the valve and press the pedal. If this is correct, then I suppose fluid is just going to go all over the engine compartment?

 

If I bled the fronts first, which "side" of the differential valve would I open? The shop manual says the "opposite" which I guess means the rear brakes.

 

If anybody has done this before, I'd sure appreciate some words of wisdom or answers to these questions if you have 'em.

 

Thanks!

Tom

[d.reese 13]

I have never had to do it, but look at this forum link at the last to comments, seems to me to be a two person job? Hope it works.

 

http://www.ford-trucks.com/forums/113549-centralizing-brake-pressure-differential-valve.html

[SlimeGold 69 15]

Hmmm..I have never done that either. When you say 99.5% sure this is the problem does the other .5% include the possibilty that a wheel cylinder is leaking or the master cylinder is bad? Maybe that idiot light is not so dumb and it knows you don't have enough pressure. I would try bleeding some more but keep in mind the .5% factor if you have not checked out the entire system. Does the brake pedal feel soft? Are your brake shoes adjusted out?

Edited January 30, 2010 by SlimeGold 69

[LoneWolf2U 136]

Yes its messy and a two person job.

 

Place rags under the master cyl.

loosen one brake line slightly. As helper turns ign key to on then depresses brake petal SLOWLY while watching the light.

When it goes out stop pressing petal and close brake line fitting.

 

If the light will not go out repeat steps with other line.

 

If there is an easier way let me know.:blink:

[Grabber70Mach 107]

This might help you out.

 

http://www.1969stang.com/mustang/for...=shuttle+valve

[foothilltom 33]

Thanks a bunch, fellas. For whatever reason, I'm just not that great at searching the archives here, so thanks for digging that up. It's just what the Dr ordered. I'll take a shot at this over the weekend with the boy playing the lovely assistant.

 

And to Slimey, I left out a very important detail in my original post to explain my certainty...if I unplug the little connector to the differential valve, the light goes out. I have already replaced the rear shoes, cylinders and the master cylinder. The car seems to brake pretty dang good, so I'm suspecting the ball valve is just off center from all the monkey business.

 

If I can't get it to go out, I'll certainly look into all the other places the warning light can come from. Thanks for the extra advice.

 

Tom

[SlimeGold 69 15]

Thanks a bunch, fellas. For whatever reason, I'm just not that great at searching the archives here, so thanks for digging that up. It's just what the Dr ordered. I'll take a shot at this over the weekend with the boy playing the lovely assistant.

 

And to Slimey, I left out a very important detail in my original post to explain my certainty...if I unplug the little connector to the differential valve, the light goes out. I have already replaced the rear shoes, cylinders and the master cylinder. The car seems to brake pretty dang good, so I'm suspecting the ball valve is just off center from all the monkey business.

 

If I can't get it to go out, I'll certainly look into all the other places the warning light can come from. Thanks for the extra advice.

 

Tom

 

Sounds like you have it covered then. I have never had problems with the valve other than dirt but that's not to say the valve ball is not your problem. You may still have some air trapped somewhere also. Did you bench bleed the master?

[foothilltom 33]

Hey Slimey, now I'm not so sure at all that I've got it covered. I bled each the 2 drum and disk brakes, but I definitely didn't bleed the master cylinder. In fact, I don't see any kind of "bleed screw" on the master cylinder at all. I replaced the master some months ago, filled it, bled the brakes as described, and so forth. Later, I replaced the brake cylinders as they seemed to be "stuck". Bled again after that.

 

So...if you have any pointers on how to bleed the master cylinder, I'm all ears!

 

Tom

[MAD IN NC 21]

Tom, whenever you change the master - you have to prime it first aka bench bleeding it.... The kits at the auto store cost $3.99 for fittings and hose's

 

http://www.misterfixit.com/brakbld1.htm

 

 

Installing a new master cylinder requires a bench bleed to ensure all the air is removed from the unit

 

After successfully rebuilding a master cylinder (or when fitting a new one) it is a good idea to bench bleed the MC before installing it in the car. This will fill the MC with fresh air-free fluid and in effect "prime" it for integration with your car's hydraulic brake system, making your on-the-car brake bleeding a little easier.

 

The basic idea is to create mini hydraulic system on your bench. You can use old brake line fittings if you have them but I didn't so I purchased a master cylinder bleed kit from my local auto parts store. A new MC may include the necessary parts already. The kit should consist of a number of plastic fittings which are designed to fit in the outlets (usually two, front and rear) of your MC. One end of the fittings will be threaded and the other will have a round smooth hose adapter. Thread the appropriate fittings into the outlets on your MC. The kit will also contain a length of plastic hose. My kit had black hose but I found some spare clear hose and used it instead - this will allow viewing of the air bubbles passing through the hose. My kit also had a plastic clip used to hold the two pieces of hose together and clamping to the edge of the fluid reservoir.

 

Clamp the cylinder firmly in a bench vise so that the cylinder area is level. If it is pointing upwards the air will remain in the cylinder. Slide the hoses onto the fittings. Cut the hoses just long enough to reach into the reservoirs and remain submerged - the shorter the length of hose the better. Place the other ends of the hoses into the fluid reservoirs (you'll probably have hold them in place somehow because once you start pumping they'll want to flail around in the air and spray brake fluid everywhere). If you can get a helper that is ideal.

 

Fill the reservoirs with new brake fluid, and pump the piston slowly and evenly, full strokes. I used a big Phillips screwdriver because its tip doesn't damage the piston and the handle gives you something to lean against. I would not worry about the fluid getting recirculated because it is brand new and you are creating a temporary hydraulic circuit with the hoses which will not become contaminated with dirt. The air which is still in the system at this point will be bled out. Pump the cylinder until the tubing contains no more air bubbles and no new ones emerge from the MC on the down stroke. On my MC this took about 15 strokes some may require more, some less. Keep going until the air stops as this will make the task of bleeding the brakes in the car much simpler. When all the air is out, mount the cylinder in the car. Here you have to be careful to prevent the fluid still in the hoses from spaying your car and any other painted objects nearby - brake fluid is a great paint remover! If you decide to remove the hoses before installing on the car, make sure to plug up the fittings - I just held the hoses up while transferring from bench to car. Once the MC is mounted in the car, remove the fittings and connect the brake lines. You'll lose a little fluid but the check valves in the cylinder should stop any major leakage. Now you are ready to bleed the brakes in your car and it should be a lot easier than if this step was avoided.

[SlimeGold 69 15]

+1 ^ also when you get it all together never let the master go dry while bleeding your brakes! I know that is a obvious no-no but believe me, it can happen pretty quick and suck in all kinds of air.

 

Also Tom before in a couple of posts up when you say you unplugged the connector and the light went out, it will. The light is lit up only by the valve switch. With a dual braking system if pressure is lost on one leg, the light lets you know, like if you blow a rear wheel cylinder for example, you may not realize it and keep driving around with only front brakes. If air is in your master, it could have the same effect.

Edited January 31, 2010 by SlimeGold 69
added more info

[jag 10]

In the mid seventies I had that happen to me. The brake line rusted thru and that caused a pressure difference. The ball then shifts to the lower pressure side. That puts the light on. Once the problem of what caused the low presure is solved and all air is bled out, the ball should re-center it self when you push on the brake pedal. There is always a chance that since the valve is now 40 + years old, it may be stuck and may need to be replaced.

JAG

[Jayru 17]

Tom, did you ever get this straightened out?

 

When my driveshaft snapped the other day, the in dash brake light came on. I figured the DS had taken out the intermediate brake line tripping the light.

 

To my surprise, the line was just fine and there were no leaks in the system anywhere. After reading this thread, i tried every which way to bleed the valve back to center, but the light was still on. It would get dimmer or flicker every now and then, but remain lit. I removed the switch, pushed the plunger a few times and now the light is off and won't come on no matter how hard i push the plunger. The switch is reinstalled and the light is out...

 

I'm wondering if the switch was just faulty and tripped with the jolt of the DS snapping??

 

Now that the light is out, how can i make sure the brakes are functioning properly? Both the front and rear brakes seem to be functioning normally (applies pads/stops wheels when brake pedal is pressed). If the valve was in fact stuck one way or the other, my front or back brakes wouldn't be working correct?

 

Am i ok?

[foothilltom 33]

Hey Jayru, I did *not* solve my brake light mystery. I'm reasonably sure my ball valve in the distribution block is just "stuck" as this car sat for 20+ years, but I may be totally wrong.

 

I've replaced the MC, Power booster, brake cylinders, shoes, etc. and the car stops just dandy. I wimped out trying to replace the distribution block because 1) I couldn't remove the damn brake lines without bending them (they were stuck), and 2) I didn't want to take on the re-routing of brake lines by buying a new distribution block.

 

I'm uneasy about this, but I just disconnected the light switch from the distribution block.

 

To answer your question about function, I just pulled the wheels and made sure that the front and rear brakes where "moving" with appropriate pedal pressure. That combined with a road test was enough for me...but again I'm not sure if I'm overlooking something important by not replacing the distrib block.

 

Tom

[Jayru 17]

Thanks Tom. Yea, I'm uneasy about it as well. But as far as i can tell the brakes are functioning normally. I was anticipating the same issues when contemplating replacing the distribution block.

 

If i ever have the motor out again (fingers crossed it won't be for a while), i may try to rebuild the factory one i guess.