Update on Brake problem

Doc 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 Respectfully Rolf

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. Rolf

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 find one...........

Hey, what about nozel's advice? Seems to contradict yours...................

Thanks so much,

Doc

Yes, I get lots of fluid from all four corners when bleeding with key off. Haven't tried with key on though.

Doc

Ok Rolf, that's the exact opposite of what others have told me (hold pin IN not OUT). So what shall I do.................I think I'm gunna try both............

Doc

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.

The kh book I have only deals with the electrical portion of this abs system.

Most recenter automatically when the system is working properly. I remember reading about some that could only be reset by removing the switch and manually moving the plunger back to the center positon with a small screwdriver.

Not related to the proportioning part of this valve. Only affects the metering or "hold off" valve for bleeding the system correctly. See earlier post for more info on this subject.

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. B

Hey, you all want to bet on how many gallons of brake fluid Doc has gone threw so far? I bet 1 and a 1/2 :)