Hot Rods Question about a brake proportioning valve

I am working on a Buick station wagon. I bought it ,it had no brakes at all. All the brake lines going in to the brake proportioning valve were rusted thru. I have replaced them all and when I try to bleed the rear brakes i just get a dribble or nothing at all. I read that a button pops up on the proportioning valve when the master cylinder is empty on one side. This prevents the rest of the brake fluid from leaking out on the other side of the master cylinder.Then it says you have to take the brake line off closest to the button and go in with a small needle or screw drive and pop a little piston back up that drops when you run out of brake fluid so the remaining front brakes will still work. Any body ever heard of that? Thanks Bruce.

 

Yes, there is a small piston like valve inside the unit and it shuts off the line that is bad. I have tried to move them back in place but sometimes it is hard to do.

 

Yep.
They stay in one place for many many years.
A line brakes and the hydraulic pressure forces the shuttle valve over.
The can get stuck because the shuttle valve was literally pressed in over to its new location.

 

SO uncleandy.What should I do?Thanks. Bruce.

 

Get a new one, many of the repruction parts houses sell them.
The same unit was used on many cars for many years, there are different versions though so get the right one.
Yours probably has a ground wire on it for the brake light too right?

Sometimes you can open the lines on the flowing slide and pounce on the pedal with non flowing side closed and that will move the shuttle valve back over.

It's a really neat fail safe device that can save people when a line failure happens.

 

No, it does not shut anything 'off' at all. There is a little piston that pressure, or lack of, moves to one side or the other to ground a circuit and trigger a warning light, it has no effect at all on the brake system or brake fluid transfer.
The rear circuit just gets a little squirt of fluid, that is normal. Bleed it with a vacuum bleeder.

 

Shut off as in completely seal, no it does not, but it does close up the gaping hole from a broken line and severely reduces the size of the passage to that of a small leak.

If you're bleeding a system with one of these, it's a good idea to put a block of wood under the pedal so you don't inadvertently move the shuttle valve close off the side you are opening. Vacuume bleeders are made for those who do not understand how this thing works. A vacuume bleeder will not activate it.

 

We always cracked the brake bleeder on a wheel cylinder on the good side (side that didn't leak), and then pushed the brake pedal down until the brake light on the dash went out. This re centered the proportioning valve. Then re tightened the open bleeder, and re fill the master cylinder.

 

That works if there's pressure and no air in the side you've repaired or have closed. A real bugger to get enough fluid past the shuttle.

 

Agree ^. The pressure differential switch simply turns on a warning light. It would be a really stupid idea to shut off any available braking because of low pressure. The divided master cylinder fluid reservoirs prevent any fluid loss when one system fails, not the warning switch.
There were three variations of the warning light switches that I know of and have info on. Some were both stand-alone or part of a combination valve, including proportioning and sometimes metering.

With centering springs--These center automatically when the brakes are released.
Without centering springs--These stay on until the pri./sec. pressures are equalized by bleeding.
With dual pistons--This design requires the switch to be removed to allow the pistons to center.

As already stated, get a new replacement switch designed for your vehicle if the lines were that rusted. The master cylinder should also be suspect.

 

As said above, it is simply a switch to activate the light, nothing else...


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At a minimum, one from a car of equivalent weight, that is (one) of the criteria for sizing the internal valving.

 

Just study the picture.
Better yet pull one apart, rebuild it.
Push the shuttle to one side and blow thru it. You'll understand.


All those who have done so stand and say "eye"
All those who haven't say " it's just a switch"

 

Not all of them are resettable; I ran into one on an AMC product that had to be replaced if pressure was lost on one side or the other. Didn't figure it out until I got ahold of a factory manual that stated it wasn't resettable and had to be replaced. In this case, it blocked pressure to one front wheel so the car would pull violently to one side when the brakes were applied, and it was a sealed unit. The owner had disconnected the warning light (!?!?) when he couldn't get it to go out....

 

Do you remember what year the vehicle was? AMC, IHC and Kaiser/Jeep used the two-piston design that requires the switch to be removed before bleeding to center.
Having one front working during a pressure differential sounds like a cross-split system vehicle.

 

How does the wire for light work? Is it a ground or hot? I assume that it is ground.

 

Delivers a ground when activated.

 

It was a early '70s Hornet IIRC. I was unemployed from my electrician job at the time so was picking up extra money fixing stuff for neighbors/friends. The owner put new brakes on and didn't remove the valve before bleeding. As I mentioned, he also disconnected the warning light for some reason when he couldn't get it to go out. The valve had a single inlet for the front brakes but two outlets, one to each front wheel. The outlet to the passenger side was blocked, so when you applied the brakes it wanted to go into the other lane. The idiot had been driving it like that before his wife made him bring it to me. The valve was a sealed unit, and IIRC was a one-year-only deal that was NLA. I ended up removing it and re-plumbed it. It was a drum brake car, after I did that the brakes worked fine....

This guy had a real knack for finding cars with weird issues. He brought me a Volvo wagon for a water pump that drove me crazy trying to get it to seal. It had a large o-ring on the top of the pump that sealed against the head. I could not get that to not leak... Finally found out that was normal, you had to get the car up to temp then the o-ring would expand and seal.

 

I would get a new one. That's what I did and everything worked as it should.

 

Well, I put on 2 new front calipars since I broke the bleeders off on both of the old ones, Bled the system again and now I have brakes. I think it s only front brakes since I am still getting very little fluid from the rear brakes when I bled them. I will see if I can get a new proporting valve. I also replaced the theromstate and the old one was covered with mud. I flushed out the cooling system and it was nasty. I looked in to the radiator after the new thermostate opened up and the water wasn't moving , so I am going to pull the radiator and have it flushed and cleaned .Seems like every wagon I find has been sitting for a while. LOL. Bruce.

 

Bruce
I know there is a generic sequence for bleeding brakes but some oem mfgs have a specific sequence on individual models, I remember seeing diagrams in shop manuals.

 

Ok, so if by your own words it doesn’t close the passage off completely he should still have pressure there, something about pascals law...unless I misunderstood your post. There are myths that continue to be perpetuated on the Internet and this is one of them. But hey I may be one of those who don’t know what I’m taking about.

 

Open a front bleeder and stomp on the brake real hard and fast. this will make the valve shuttle to the other side.
Now when you bleed the rear brakes make you don't apply some much force the the valve goes back and cut the fluid from the rear again.

 

Trying a different tactic here, IF the pressure differential valve blocks fluid in the event of a leak can someone explain to me the purpose of a tandem master cylinder?


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Check out this video from Master Power Brakes. It explains it so it all makes sense.

 

Not sure on the proportioning valve deal so I won’t get into that argument. BUT, it sounds like the car hasn’t been in use for quite awhile, so do yourself a favor and replace the rubber also. I’ve had them break down and not pass fluid....

 

There you go

 

Ok this whole thing prompted some more research on my part, I’ll admit I had no knowledge of a pressure differential valve that blocked fluid flow as well but master power brakes says theirs does but not a factory system, the only other place I saw that referred to was in studebaker service info that called it a special pressure differential valve. I stand corrected that it is possible but maintain the majority of FACTORY switches are for the light only. I apologize for any post that came across short but I have learned something here! Never a bad thing


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Running thru Bruce's first post gives all the clues to what he's working on and almost exactly what all the brake lines are connected to. The symptoms he described are exactly what to expect when that shuttle shifts over. Being familiar with it helps recognize it as such.

When a bleeder is open the pedal goes to the floor does it not? when a line is open the same condition, pedal to the floor yes? However when that valve is present and the shuttle has shifted you will have a pedal that DOES NOT go to the floor. When bleeding,The amount/volume of fluid passed through is minimal, so little fluid (like 3 drops from an eye dropper) that there is no pressure development long before someone says WTF.

It's a very neat valve. Brilliant idea. So awesome it can't be , but it is.
The explanation doesn't sound exactly stupid either Since there's no available braking with an open line.