"Centering" the combination valve?

[19GMC63]

Whats the proper procedure to do this? I'm not getting fluid to the rears, the lines are clear, the MC is bled, so it has to be that stupid combo valve.

I've heard so many different ways of "centering" one.

Just in case, heres a "rusty" pic:

[19GMC63]

anyone?

[SCOTI]

I would like to know this as well. Is there an official GM 'tech' method for this??

[BACKYARD88]

Got this from CPP. Hope it helps.

Test Combination/Proportioning Valve

Use a test light by attaching a clip to a positive contact on the vehicle and touch the point of the tester to the electrical connection of the combination valve. If the the light does not come on, the valve system is operating correctly and no further testing is required.
If the light does come on, this indicates that the pressure differential valve is stuck in the front or rear position.
Bleed the brake system to determine if the front or rear lines are blocked off. Set up one front wheel and one rear wheel for bleeding at the same time. Crack both bleeder screws and gently pump the pedal a few times.
The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal pressed. An unblocked line will squirt fluid out the bleeder.
The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.
Slowly press the pedal with steady pressure a number of times until the light goes out; this will center the differential valve. You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out, close the bleeder screw.
Heres the link:
http://www.classicperform.com/TechBo...oubleshoot.htm

[19GMC63]

thanks for the link, this procedure dfinitely makes sense.
Will try this today.

[SCOTI]

Originally Posted by BACKYARD88

.... Crack both bleeder screws and gently pump the pedal a few times.

The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal pressed. An unblocked line will squirt fluid out the bleeder.
The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.

My experience w/fluids say the brake fluid will follow the path of least resistance if one port is open & the other closed. How will that allow the system to build pressure where it's needed?

[BACKYARD88]

My guess is that this procedure is supposed to build enough pressure on the closed side to free the valve since it won't be able to build any pressure on the open side. This is just a guess as I have never encountered this problem myself before.

[PanelDeland]

Scoti,the front wheel cyl use a different reservior than the rear.If the front wheel cyl is open it wont build pressure but the rear will and it pushes the combination valve to center it.

[Huck]

According to the GM shop manual, you need to have a "tool" to push the front pin "IN" when bleeding the brakes on our trucks. I have used a big C clamp to do this--but then again, I have replaced the master cylinder, the proportioning valve, the calipers, the rear wheel cylinders----the booster and still the brakes on my blazer SUCK!! Even with everything new, bleeding the brakes etc. ---my pedal goes just about to the floor before the big POS stops.
Kind of getting a little sensative on the brakes issue as this is one big PITA I am tired of dealing with. About ready to convert to a newer style system. I have paid enough money to buy 2-3 2007 systems!!

[LostMy65]

Originally Posted by PanelDeland

Scoti,the front wheel cyl use a different reservior than the rear.If the front wheel cyl is open it wont build pressure but the rear will and it pushes the combination valve to center it.

Yep.
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[LostMy65]

I've been fighting mine. So much that I was tempted to dump it and go with just an adjustable proportioning valve.
But then I read some really good info on the superiority of having the combination valve with the warning light and metering. The combination valve supposedly has the ability to 'meter' the front and rear needs for different braking circumstances.
If I was driving 30 and gently applied the brakes, the weight shift to the front wheels is a lot less than doing 80 and slamming on the brakes.
Each case would require different pressure to the front and rear. An example using arbitrary numbers would be as an example of gently applying brakes would be 60/40 (front/rear) But when you slam on the brakes, more weight is shifted forward to the front wheels. Meaning the rears will lock up at even less fluid pressure to the rear.
Supposedly, the metering valve can adjust for different situations.
Whether it does or not, the physics of varying required rear fluid pressure makes sense.
So eliminating the combination valve and replacing it with an adjustable one isn't recommended.
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[cdowns]

i take a cclamp and tighten it down on the pin