Topic: Brake service light stays on

[AGUHR]

I have an issue where my brake service light will not turn off.
Fluid is full
Lines have been bleed
what else could be causing the issue?

I've pulled up on the e-brake pedal. which BTW is not hocked up, so i will zip tie it up so it does not fall down again or take it completely off. Nice about FL no inspection laws.  One day I put it all back on but not right now.

I know when I unplug the circuit under the proportion valve the light goes out - if that helps.

Brakes feel tight, i stop just fine.

[VA_YJ]

"I know when I unplug the circuit under the proportion valve the light goes out"

I think you've already found your problem.  Jeep calls the proportioning valve a "combination valve".  You should check for leaks -  rear wheel cylinders, front calipers, hoses, lines...

I had a proportioning valve go bad on a Ford once - the o-ring the switch valve went bad (with no leaks in the system).

From the FSM:

A combination valve is used in all models. The
valve contains a pressure differential valve and
switch and a rear brake proportioning valve. The
valve is not repairable. It must be replaced if diagnosis
indicates this is necessary.
The pressure differential switch is connected to the
brake warning light. The switch is actuated by movement
of the switch valve. The switch monitors fluid
pressure in the separate front/rear brake hydraulic circuits.
A decrease or loss of fluid pressure in either hydraulic
circuit will cause the switch valve to shuttle to the low
pressure side. Movement of the valve pushes the switch
plunger upward. This action closes the switch internal
contacts completing the electrical circuit to the red
warning light. The switch valve will remain in an actuated
position until repairs are made.

[AGUHR]

Can the pressure differential switch under the combination vlve be changed out? I looked for it on NAPAonline and could not find it. Or is it part of the actual combination valve setup and must be purchased together?

[VA_YJ]

There are two possibilities here: 1) you have a leak in your brake system that caused the light to come on 2) the combination valve is jacked up.

I would start by checking for leaks.  Sometimes, the rear brake system can leak and you barely notice it.

The service manual says the combination valve is not repairable.  That means you probably won't find any repair parts.  To some of us, the term "not repairable" is a challenge.     You can take it apart and look for problems (cut or worn o-rings, scoring on machined surfaces, etc.).  Just remember that any replacement parts you put in there must be able to handle brake fluid.  It's probably easier to just replace the whole valve.  Make sure you don't have any leaks (e.g., rear wheel cylinders) or the light will come on again.

[AGUHR]

No leaks that I can see. I've upgraded all of my axles. Not sure if that matters any.
Front I have a 79 F150 HP D44 with the stock F 150 calipers (new of course)
Rear I have a 98 explorer 8.8 disc brakes.

Since I haven't had any issues in the past with these axles I'll pick up a new combination valve.

[Bounty Hunter]

Upgrade to one from a 4whl disc setup, I think the one I used is from a Grand Cherokee.  The 4whl disc proportioning valve is different as it doesn't hold residual pressure to the rear as it does for drum brakes.

[AGUHR]

Are proportioning valves only available at dealerships? my local NAPA and Advanced Auto both said its a dealer item only.  I also assume the circuit on top of the proportioning valve can not be purchased alone?

I'm trying to get it all fixed before next weekend - Jeep Beach in Daytona and I dont want to drive across the state with my 4 year old and have something "brake" on me. (a little play on words therre - you see that  )

[VA_YJ]

Mopar part numbers for the Grand Cherokee combination valve:

5200 9129 ZJ w/Rear Disc Brakes 1994
5200 9061 ZJ w/Rear Disc Brakes 1995-96

Maybe you could pull one from a wreck? 

[AGUHR]

Time is a factor for me, I work 8-7 normal days. Called the local dealership - $151 for a new one - will be here in the morning. Hopefully this is it and its not a gremlin.

[AGUHR]

The only place I could find the part was the Jeep dealership - $151.  So I rolled the dice this was the issue and bought the proportioning valve for the $151.
Low and behold this was the issue. Once the air was bleed out of the lines the light went off.
Funny thing is I do believe it coasts better as well, not sure if im imagining things or what. But it works and all is good again in the world.

thanks for all the insight.

[neale_rs]

Glad to hear it is ok now.  Did you get the proportioning valve for rear disk brakes?

[AGUHR]

I used the stock version. I've had rear disc for 3 years now. This is the first time I've had an issue. I guess I'm playing my card and hoping it won't happen again, if it does I'll probably look into a different setup.

[neale_rs]

The valve for rear drums holds residual pressure to the rear brake line.  I thought that maybe it was coasting better due to having gotten one for rear disks that does not hold the residual pressure that can cause some brake drag with rear disks.

[sharpxmen]

i didn't think they hold pressure (but i don't know for sure to be honest), i thought the reason you replace the prop valve is for front/rear ratio (hence the name) due to the rear calipers having different clamping force compared to the pistons for the brake shoes in case of drums.

[neale_rs]

You got me doubting so I did some reading and it seems the master cylinder will have a residual pressure valve where the drum line connects, not visible from the outside.  The proportioning valve itself will not have it.  Sorry for the bad info before.