YES, but.....I dont know if it will center the piston in there or not by
doing this. Remember, your brake light will not be on if it is right.(must
be plugged in). Try to remove the switch or take the whole thing apart if
need be to center it. Then bleed while keeping the pin out. Do you
understand the piston part? do you get LOTS of fluid from all the corners?
Whenever work on the brake system is done it is possible that the brake
warning light will come on and refuse to go off when the work is finished.
In this event, the switch must be centered.
1.. Raise and support the truck.
2.. Attach a bleeder hose to the rear brake bleed screw and immerse the
other end of the hose in a jar of clean brake fluid.
3.. Be sure that the master cylinder is full.
4.. When bleeding the brakes, the pin in the end of the metering portion
of the combination valve must be held in the open position (with the tool
described in the brake bleeding section installed under the pin mounting
bolt). Be sure to tighten the bolt after removing the tool.
5.. Turn the ignition key ON. Open the bleed screw while an assistant
applies heavy pressure on the brake pedal. The warning lamp should light.
Close the bleed screw before the helper releases the pedal.
6.. To reset the switch, apply heavy pressure to the pedal. This will
apply hydraulic pressure to the switch which will re-center it.
7.. Repeat Step 5 for the front bleed screw.
8.. Turn the ignition OFF and lower the truck.
Are there bleed screws on the valve? If not bleed at the lines on the valve.
What you are trying to do here is use pressure to center the valve's piston.
Dont confuse rear brake bleed screw (in #2) with the rear wheels, we are
talking on the valve here. Just watch the light for confirmation on reset.
Allright! Now we're talkin'.....................
No, there aren't any bleed screws on the valve that I remember seeing. I'll
take a closer look when I get home from work; maybe they're located in the
back somewhere out of sight. If there aren't bleeders on the valve, it's
gunna be impossible to rig up a hose! Maybe I can just crack the lines at
the valve and use a shop rag to catch all the fluid.
Number 4 is kinda fuzzy too. Do I need to loosen the bolt on the prop valve
before I give the pin a tug or can I just pull her right out with the bolt
nice and snug? I don't have the tool described and have NO idea where to
Hey, what about nozel's advice? Seems to contradict
Thanks so much,
The combination / proportioning valve 'pin' should move out with pedal force
applied and more importantly retract when pedal is released.
This indicates front brake is pressurised.
To recenter light, block the pin from extending (gm or snap on or Mack
probably have such a fixture) then push hard on pedal to re seat switch.
Be careful though for you could set the switch to rears failed position.
The pin protruding from front side of valve is not the switch trigger:
Trigger is buried deep inside the assembly.
Proportion/ combiation valve is probably toasted due to its age
I'm reading this thread with great interest. I have to replace the flex
lines at the front of my 90 Sierra1/2ton. And possibly calipers depending on
the bleeder screws. I'm not going to dive into it yet till I see how this
one goes, heh.
Don't hold pin out, rather prevent pin from popping out whilst bleeding
Gm manuals have a tool number but snap on or Mack may be able to supply
fixture and is recommended if power bleeding your system.
That pin is the metering valve bypass for the front brakes. It holds off the
pressure to the front until the rear brakes have a chance to start working
(have to overcome all the return springs in the drum brakes before you get any
braking). If you can see the pin, it needs to be pulled out. If its covered
with a rubber nipple, It needs to be pushed in. It should be bypassed to bleed
the brakes properly.
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