need help with combination valve

[faribran]

we have combination brake valve that appears to be malfuctioning.
It worked fine until the rear end was changed and the master cyl was emptied.
When the brakes are applied the plunger protrudes from the front of the valve..
the light is coming on .. and the brakes are not as good.. we have bled, but no luck...

any suggestions?

[ChevLoRay]

I think there is a procedure to use that centers the valve into position to give equal braking on fore and aft. Try to find information about that procedure and I think it will help, when you get it accomplished.

[hyarbour]

There should be a metal "button" on the proportioning valve that needs to be pushed in during the bleeding process. I use a bent piece od steel and a pair of vise-grips.

[faribran]

Ttt

[Orange]

When you bleed, what happens?

No fluid in the rear or what?

If you drained your M/C then you may have air locked it. You probably need to re-bleed it anyhow, because there is no doubt air in it. (Just "bench bleed" the rear line side on the truck).

DLB

[faribran]

ttt

[JimKshortstep4x4]

Bleeding the rear brakes can be a problem so I believe that there is still air in the system. This causes the valve to stay off center. When this problem pops up on rear brakes, I start bleeding by removing the lines to the brakes from the valves. Once the lines are removed, I connect two hoses to the valve and bleed back to the master cylinder. This will bleed the master cylinder and the valve. Also, it will confirm whether or not you are getting fluid past the valve, (I have never had to clamp the sliding valve in the prop valve).

If the bleeding works at this point, reconnect the lines and rebleed the rear wheel cylinders. Because of the length and diameter of the rear brake line bleeding can take a while.

Make sure that the rear shoes are adjusted so there is not too much movement in them. I adjust the rear brakes by running the adjuster up until it locks the drums and then back off until the drum will turn with a slight drag.

Jim

[faribran]

thanx Jim.. sounds like some good info..... will try a couple of those things...

[GMC AMI]

When bleeding, RUN to a first aid station! JKing. Start with the the rear wheel
on Pass. side then drivers side. Like stated it will take a bit of pumping the petal to get all the air out of the rear lines. Good luck and happy pumping, just love the pumping part!

[cdowns]

according to factory service manual, which i guess most dont have and even less read the pin MUST be clamped in on the proportioning vale to properly bleed the brakes//there is of course a special gm tool to do this but ive always just used a cclamp

[SCOTI]

Originally Posted by cdowns

according to factory service manual, which i guess most dont have and even less read the pin MUST be clamped in on the proportioning vale to properly bleed the brakes//there is of course a special gm tool to do this but ive always just used a cclamp

Although this sounds easy enough for the 67-72's w/the prop valve dangling under the master cylinder, how the heck do guys w/73 & later trucks access the prop valve for this procedure?

With all the brake lines, pwr steering lines, shroud & fan in the way..... good luck getting something on the end of the valve to depress the 'pin' & hold it there while trying to bleed the system.

I purchased a 'new' replacement prop valve for my 68 & wound up installing it on my 74 instead (my 68 uses 74-76 hard brake lines/prop valve). Rather than getting another & worrying about this, I think I'm going to get an adjustable valve & dial in the bias myself.

[faribran]

Cdowns..... thats interesting.......so a c-clamp can be clamped along the length of the proportioning valve to hold the pin "IN" while bleeding?....

this is why i really like this board....... good input from many good people..

thanks... my son is bringing the truck home this weekend.. we will try this.