M1008 Brake light/rear disc conversion

[firstcucv]

hey guys i am in need of some help. I Did a disc brake conversion on my cucv's 14bff because i had a leaking iner seal on the pasenger side. the driver side drum looked vary bad it was groved all the way across the shoe. The conversion went well till i tryed to test it out the rear caliper pistions stayed out after the brake's where applied. this was friday. i started to look at it today and found that when i opened the bleeder on the driver side it relesed both rear caliper pistions. i started the truck to try it again and the brake light came on. also the calipers hung-up again.

any ideas would help! i dont even know what would make the brake light come on.

 

[Warthog]

There is a combination vavle on the front crossmember that the brake fluid travels thru. If one of the circuits (front or rear) goes out, the fluid pushes a switch inside the valve alerting the driver that there is a brake issue.

When you opened the bleeder, the switch was tripped. There really isn't a way to reset the switch except letting it do itself. The TM covers how to not trip it but not reset it.

TM 9-2320-289-20, paragraph 2-25 and 7-6

Why the calipers are hanging is another issue. What "kit" did you use? Did you replace the rubber brake hose?

 

[firstcucv]

I repaced the line fron the frame to the diff last year when i lifted it and i replaced the lines from the diff to the now calipers when doing the conversion. all are From ORD and are flex lines. The conversion Came from DIY4X and the calipers, rotors and pads ar from a 86 k20 4x4 front. all parts are reman or new.

 

[Skinny]

Won't the combi valve reset if you jab the brakes really hard a few times? Unless you can manually reset the plunger, I think high brake fluid pressures causes it to reset after a line repair. Could be wrong...

 

[Sharecropper]

I would say to forget the light for the time being and focus on getting the calipers to work properly. The valve will reset itself when all pressures equalize.

I installed rear discs on my M1028 http://www.steelsoldiers.com/cucv/52121-rear-disc-brakes.html but did not experience any problems such as you describe. I am at a loss at what could be the problem.

 

[firstcucv]

I would say to forget the light for the time being and focus on getting the calipers to work properly. The valve will reset itself when all pressures equalize.

I installed rear discs on my M1028 http://www.steelsoldiers.com/cucv/52121-rear-disc-brakes.html but did not experience any problems such as you describe. I am at a loss at what could be the problem.

I think i agree the light is the lest of my worys. i read your thred at the end some one mentions the rear load valve and removeing the ball in the propotioning valve? could any of this help or hurt me?

I tryed moveing the load Valve(think thats what is called) all the way up and down didn't change anything.

Other wise my set up looks just like yours.

 

[ODdave]

Loosen the steel line where it goes into the rubber line. If it dose not release the brakes , replace the hose.

 

[firstcucv]

Loosen the steel line where it goes into the rubber line. If it dose not release the brakes , replace the hose.

I did this and it does the samething as opening the bleeder. Think i am going to try this on the line leeding to the rear Load valve. Is that valve needed? what happens if i take it out?

 

[Sharecropper]

The rear proportioning valve plays an important role in how much braking force is sent to the rear wheels and should not be removed. As weight is added to the bed, the arm connected to the rear axle causes the valve to rotate counterclockwise, therefore increasing the braking force. Correct adjustment procedures are detailed in Post #75 in my rebuild thread.

 

[firstcucv]

Ok i split the line leeding to the rear at the joint under the cab on the pasenger frame rail and it did not let the brakes loose. my guess is there is somthing wrong with the load valve.

 

[MarcusOReallyus]

The rear proportioning valve plays an important role in how much braking force is sent to the rear wheels and should not be removed. As weight is added to the bed, the arm connected to the rear axle causes the valve to rotate counterclockwise, therefore increasing the braking force. Correct adjustment procedures are detailed in Post #75 in my rebuild thread.

Hmmm.

That would imply that installing adjustable air bags to take some of the load would mess up the calibration of the proportioning valve. Since the air bags would prevent the bed from settling down as much, the valve arm won't get moved as much as it should, so you'll get less braking than you should for the load.

Is that correct?

 

[Sharecropper]

It wouldn't mess up the calibration of the valve, but instead it would prevent the valve from doing what it was designed to do.

Think about it: If you place 3000 pounds in the bed and are traveling at highway speeds, and then suddenly have to stand on the brakes for whatever reason, you want the rear axle to be the dominant braking axle. If not, the momentum of the load in the bed will cause a loss of control. Very similar to pulling a heavy trailer. I always adjust my trailer brakes so that they are doing the bulk of the braking action. If not, during emergency braking the trailer will want to come around the truck. Several years ago I was pulling a 4-horse trailer 60 MPH on the interstate when all of a sudden the trailer ball broke off! The safety chains caught the trailer and I jammed on the trailer brake lever without touching the truck brakes. The trailer wheels locked and I slowly came to a stop on the shoulder of the road with full control. If I did not have those trailer brakes, I probably would not be typing this.

So the rear propostioning valve acts in the same manner. The more load you add to the bed, the more braking power you want at the rear axle.

Hope this helps.

 

[sht2kll]

Howdy: The Rear valve is called a height sensing proportional valve GM p/n 15538215. How do i know this, I just finished a rear disc conversion on my truck. When I was finished, the rear brakes would lock up and stay that way till I opened up the bleeder on one of the calipers. Sharecropper gave me some good information about the vavle, but my valve was bad. For now I had to remove it, till I get the new one. The combination valve will reset itself, at least on mine it did. I wound up going through about 1 qt. of DOT 5. 2 of the hard lines burst due to rust. Love the new discs.

 

[MarcusOReallyus]

It wouldn't mess up the calibration of the valve, but instead it would prevent the valve from doing what it was designed to do.

Yes, that's what I should have said. The calibration is just fine. The air bags just prevent it from being relevant!

Thanks for confirming that. I've never messed with brakes, and the concept of these valves is new to me.

Anyway, it sounds like it's bad mojo to use air bags, then. When you need them the most (with a heavy load) is when you can least afford them!


Do all trucks use such a system?

 

[ALFA2]

Please take a look at a residual valve, sometimes they were in the master-cylinder, in the fitting for the rear brakes part of the system and sometimes they were in the combination brake warning/proportioning valve on the frame in the front of the truck. Look in to it, if you did not remove the residual valve from one of these places, it will cause some of the problems you described.
Hope it helps.

ALFA2

 

[Sharecropper]

Sht2kll - I had that GM part number for the valve but have been unable to locate one. Where did you find it?

 

[firstcucv]

Howdy: The Rear valve is called a height sensing proportional valve GM p/n 15538215. How do i know this, I just finished a rear disc conversion on my truck. When I was finished, the rear brakes would lock up and stay that way till I opened up the bleeder on one of the calipers. Sharecropper gave me some good information about the vavle, but my valve was bad. For now I had to remove it, till I get the new one. The combination valve will reset itself, at least on mine it did. I wound up going through about 1 qt. of DOT 5. 2 of the hard lines burst due to rust. Love the new discs.

Thank you for your exerience. I ended up buy passing my proportional valve. I have not put alot of miles on but everything works well this way for me. and the truck stops way better. I think that more people have a bad valve then they probably relize. I think my drums where stuck on the hole time i have had the truck.

 

[FireFighterHill]

NEW GM VALVE ASSEMBLY FOR CHEVROLET SUBURBAN 1500 #15538215 | eBay

Maybe see if there is a way to clean or restore the old valve.