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How do I flush,bench test, proportioning valve?



I don't want to pay $89 + $12 shippingand wait a few days to get a new Delco unit, so how do I flush and bench test the old one?
Reading a tech manual I have that details brake operations, there is no recommended procedure to bench test a proprtioning valve by the home mechanic. I'm sure that there are shops that can do it. I responded to a brake question in the "Other Car" forum in which I described the resetting procedure. If you cannot find that thread using the "Search" feature, I'll try to find it tomorrow.

Mainly, I need to flush it out. Ever done that, or know anyone who has?
Well, this isnt vette specific, but...

Usually, if it's one you can disasemble at all, you can either use compressed air to force the internals out, you may or may not have to unscrew a fitting or two. Sometimes you can get seal kits for them, you stand a chance of tearing selas if you disasemble it. Also dont let any parts come flying out and hurt you if you use compressed air.

Failing that, ahh, submerse in a can of denatured alcohol? Let it sit, swish it around ocasionally. The alcohol will dry out, just have to let it sit out for a few days.

What year vette? Is it the little block looking one with the switch that's under the hood down low on the drivers side?
Here is Bullitt's previous thread from the "Other Car" forum:
Bullitt said:
Well, if you still have dirt/rust in your master cylinder, then I suspect a crudy caliper, wheel cylinder or brake line. Was one reservoir dirty or both? The reservoir nearest the firewall supplies the rear drums, the other one nearest the radiator, the front discs. This would simplify trouble-shooting, obviously. You then could disconnect one line from one caliper or wheel cylinder at a time and have someone depress the pedal. While this will introduce air into the system, you are already having problems, so it can't get much worse. If there is no dirt, then you can eliminate certain lines. Reconnect the lines and try to eliminate the caliper and wheel cylinder, to find the suspect part.

Another problem that would contribute to poor braking is a proportioning valve. You have to bleed the system first, then recenter the valve. I'm not sure which type of valve is equipped on your Chevy. There's one type mounted near the master cylinder and a conventional remote mount, like the one on Sharks. Some pick-up trucks and like models have a variable proportioning valve, called a pressure control valve or load-sensing, but it's the same thing. Here's the general procedure for Pressure Differential Valve (remotely mounted):

1. Fill the master cylinder and bleed the brakes.
2. Turn the ignition switch to "On" or "Accessory"
3. Slowly depress the brake pedal down and the piston should center itself, causing the brake light to go out.

This should reset the valve. If it does not, you will have to have someone depress the pedal as you open a bleeder screw. You have to immediately close it, as soon as the brake light goes out or you will have to repeat the procedure, using a opposite side bleeder, whether you tried the front or rear first.

If you have a master cylinder mounted proportioning valve, make sure that a new sealing washer is used. My tech manual suggests that if the valve is suspect, diagnosis should be done by a reputable shop. They give no instructions, otherwise. I hope this helps.

Just went and got a catalog to jog my memory with some pics.

On the 74 I was able to flush and atleast verify that the internal piston moved with air preasure. I dont recall being able to actually remove the piston on it, but I may be remembering fuzzy again. I know you unscrew that switch and basicaly soak it in your choice of cleaner or oil and verify that it isnt stuck with compressed air in the ports, you may have to cap off one port and apply air to another or something like that to get the preasure right.

I honestly cant remember if I used a solvent for cleaning or not, nor can I remember if there were any rubber seals in there, or just a steel piston. If there's rubber seals that cant be removed before cleaning, I might be hesitant to use too strong a stuff in cleaning.

Hope that helps. I swear, if I could remember half what I do I'd be dangerous...

Wolf it's a 78. Yep that's where it is!
Tanks, Rare 81!
Wolf, that's te procedure I ad in mind to try. I may get on that today now that I've spent the morning cleaning the shop up!
well your doing better than me, got up nice and early and it's poring rain and cold here.. :(

Have at it and see what you find, I still almost want to say the little piston thingy is removeble after you unscrew one of the fittings, should be pretty obvious. Carefull of rounding off brake line fittings. :)
Yea; I'm about due some new flare nut wrences anyway. I could use your rain. It's about ninety and dry as the desert here. We've been on a two month drought!
Got it out and partially disassembled.

I pulled it out; it looks clean inside. One end comes off (the end that the rear brakes are connected to), and there is a spring loaded plunger visible that moves freely. After removing the brake light switch, there is a piston visible that soes NOT move freely. So I'm soaking it in Kroil overnight. I'll play with it again tommorrow.
If you carefully apply shop air to one of those ports, and block off the correct other ports, it will shoot that piston out, that's also a good way to move that non-moveing part back and forth, as I recall it's suposed to be stiff.
I'll give it a shot (of air) tommorrow afternoon.
OK, I sprung for a new proportioning valve today. Art's Corvette Parts in Ft. Lauderdale had one in stock. After disassembling the old one, I came to the conclusion that the only way to adequately bleed the sytem after replacing the proportioning valve is to back bleed the system. So now I'm making some fittings to back bleed with. The fun never ends!
You surly right about the fun part.

Bleeding is very critical with these things, if everything isnt just so with the brakeing system, it dosent seem to want to work right. Not very forgiving...

Post your progress, I always like to read other's experiences.. :)
The fronts are bled. I'm going to try to raise the rear of ther car enough that the back will fully bleed without back-bleeding them.

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