Interesting.
The bore of the valve housing is constant, end to end.
I would suspect that since pressure from the master cylinder is applied to the outermost edges of the shuttle, and it is machined in such a way to step down the diameter of it at the ends, that no, this particular design would not stop the flow of fluid.

In actuality, if these types of valves were designed to completely stop the flow of fluid on only one side of the master cylinder, wouldn't that create a hard pedal from the blockage and prevent the other side from functioning fully, if at all? Seems correct.

Also somewhat interesting, is that (at least in this application anyway) the only thing preventing fluid from coming out of the switch is those two small seals on the shuttle, as the molded plastic switch is not sealed in any way what so ever.

 

 

By studying those diagrams, it seems the disc brake version is the same.
Keeping in mind that the shuttle can only travel so far before it either hits the housing or the switch pintel is into the seal area.

The front of the shuttle is machined in such a way not to block fluid, and the rear side has a smaller diameter than the bore to also allow fluid to pass.
The various bleeder rod configurations looks to bypass the front's proportioning circuit to allow unrestricted flow while bleeding. The shuttle looks to remain functional and unrestricted in its movement.

Hats off to the engineers who designed these things.

 

Indeed! And even though I'd run without one for decades in my Bronco with just an H-block, I am figuring on installing one when I renew my system. That, or configure my system with the individual components that are available as stand-alone units. Personally I think the combination valve is more compact and therefore probably the way I'll go. But it's nice to know you can build your own custom system if you want.

Unfortunately the problem with the aftermarket combo valves has been for many years that they leak around the front metering valve for some reason. Not all of them of course, but when you have to return or send out fix-it parts to what probably amounted to 20% of the ones you sell, there's an obvious problem with the QC at some level.
Wilwood, which traditionally only sold a manually adjustable rear proportioning valves, has recently come out with their own combination valve along the same lines as the ones we see most often. The squared-off block-ish thing that you see so often and what GM used I believe back in the day. Time will tell if they have gotten past the whole leak-right-out-of-the-box thing.

For those reading this that are not familiar with this whole combination valve thing, the various functions are very clever, as knightfire said.

1. Metering (or "hold-off") Valve: Traditionally used in disc front/drum rear systems, this component delays the actuation of front brakes for a slight bit of time to allow the rear brakes to "catch up" so to speak. This helps in two ways. It compensates for any extra travel a slight out of adjustment drum setup needs to apply, but primarily it hopefully allows the rear brakes to apply first, acting as a system stabilizer to keep the vehicle from fishtailing. Think along the lines of using trailer brakes to stabilize an unruly load being towed.
1a. This is the component that you need to push (OR pull depending on the unit) and hold open while bleeding the front circuit. Most people get away without performing this step, but it causes inconsistent bleeding often enough that you should at least attempt to follow the bleeding instructions whenever doing yours.

2. Proportioning Valve: Used ONLY in the rear circuit. Never the front. This reduces the amount of available pressure to the rear brakes under a hard/panic stop situation. As I understand it they don't reduce pressure under light braking action. Only when the brakes are applied suddenly. I could have that wrong, but I believe that is correct that it works only when in a sudden stop situation. Again, to reduce the wagging the dog conditions of the rear brakes applying too hard and locking up the rears. When working correctly, you could still lock up the rears if you try, but it should be more controllable.

3. Pressure Differential Valve: This is the shuttle and switch bit that turns on the Brake Warning Lamp in the dash to let the operator know that there is a leak in one circuit that's changing the pressure seen at the valve. The unequal pressures move the shuttle to one side or the other. Up to you then to find the leak and fix it before anything bad happens.
I know mine worked very well on the '71 when at one point in my commute the light came on. Checking the system I found my rear axle hose from the frame to have developed a tiny leak at the crimp. Fixed it right away, but never was able to reset the switch without completely disassembling the unit. Very common problem unfortunately.

As said, very clever those designers!

4. One thing that is not in the combo valve but worth mentioning in this discussion anyway is/are the Residual Pressure Check Valves (RPV) that are usually found inside the master cylinder outlet ports, but can be stand-alone units in the hard lines as well. Your original master cylinders are almost guaranteed to have come with them (at least for the drum brakes) but most aftermarket master cylinders have left them out for the last 30 years or so. While some of our brake systems can get away without them, they are still pretty important. And critical if you have certain types of wheel cylinders in your truck/car. Pays to know what type you have especially if you're dealing with an older truck. Such as a '72 or earlier model.
These keep all the pressure from bleeding off when you release the pedal and maintain a few psi in the system after the master. Usually about 10psi for drums and 2psi for discs.
This pressure can keep the cups/seals in correct form inside the wheel cylinders so that they are always full of fluid. But another primary function of them was to maintain fluid in the system at all back when many master cylinders were mounted down low on the frame and could then in fact be mounted below the brakes. In those cases fluid could literally gravity bleed back into the master and give you a funky pedal when you most need a good solid pedal.
Since almost nobody mounts their masters low anymore (except custom builders perhaps), and most wheel cylinders come with internal springs to hold pressure against the pistons, I'm guessing that's why most master cylinders no longer come with RPV's installed. But if you're building your system from scratch, this would be a good time to re-introduce RPV's to your system.
So a good system would include both a combination valve like in this discussion AND a pair of residual pressure check valves.

Just thought I'd add some extra junk for those that didn't already know. And I had some spare time to type while waiting for the bank to call and tell me my ship had come in!

Paul

 

Was it confirmed that joining the 2 wires on the body side into one actually works? Will the brake light operate normally? (with key in start etc.)
If so I will need the single plug to do it with.

 

I rebuilt my entire brake system and the damn brake stayed on. It was on when I first bought the truck.
How did I fix it??

I disconnected the damn wire from the proportioning valve.

Problem solved, light off.

 

 

With all else working as expected, and you're utilizing the newer GM style single contact warning switch, then twisting the two wires is a proven method on hundreds of Early ('66-'77) Broncos.
Never tried it with a full-size, but if the wiring is the same as the EB, and you're swapping combo valves to one that has a single contact switch, then it should work as expected.

Paul

 

The WARNING LAMP was put there for a purpose.

 

Was your comment just to confirm it's usefulness, or was it directed at us for supposedly defeating it's purpose? None of this latter discussion is about doing that.
It's simply a way of wiring up the different style switch (1-pin vs 2-pin) specifically to keep it fully functional. Including the temporary illumination during START with the prove-out circuit.
Nothing at all to do with getting rid of the lamp. Just letting it work with newer/different generations of combination valve.

Wasn't sure if that's what you were trying to get across, or not.

Paul

 

Quote:

Was your comment just to confirm it's usefulness ...

Yes ...