goofy brake light/pedal

[wdkingery]

i just purchased a 1989 gmc sierra 6.2 diesel for the wife. one owner, great shape.

had a blown rear brake line. owner replaced the master cylinder, found the real leak and gave up; sold the truck.

i replaced the line, it was leaking nicely. the brake light was on.

now, i cannot for the life of me get brake pressure/fluid to the rear wheels. brakes don't work till the last little bit of the brake pedal.

if i pump the brakes while driving, the brake light will go out for a second, but eventually return.

i am reading about this proportioning valve, and it can get stuck, and it has to be reset, and if it thinks there's a problem it turns off the rear wheels, etc.. so i am thinking that's the problem. how to "reset" it? or get it to allow brake pressure/fluid to the rears?

thanks any GM techs out there!

[twaddle]

That's a heck of a compliment to pay the wife, "one owner, great shape!! hee hee

[Robyn]

The "Combination valve"
This little critter is located below the master cylinder usually near the frame.
The brake lines go through the thing and there is also a wire that connects to it.

The valve does two things, it detects a loss of pressure on either circuit and this causes a little spool valve to shuttle and that turns on the light.
This is the pressure differential valve part of the thing.

The other part is the proportioning valve, this part cuts off brake pressure to the rear wheels under real hard braking, to help prevent the rear wheels from sliding.
(actually reduces/restricts pressure)

If the proportioning part of the valve gets stuck or otherwise compromised the rear wheel pressure will be blocked off.

Early production valves had spring isues and would prematurely cut the pressure off.

Replace the valve assembly and then carefully bleed the entire system.
(Auto Parts store item)

Its called the combination valve, but some parts guys know it as the proportioning valve.

Take care to get fluid through the system and the air out before really putting pressure on the pedal.

You need to get all the old dirty fluid out of the system while your at it.

Once the new valve is installed and the system bled properly, things will work fine.

These valves are notorious for doing what you are seeing.

Normally if there is a system failure, all thats needed is to fix the problem then mash the brake real hard to recenter the differential valve spool and that will turn off the light.


Good luck and keep us posted


Missy

[wdkingery]

I had to unplug the dump valve, causing it to reset, and i got rear brakes!