Brake help

[lennyKatan]

Hi

getting my 89 comanche ready to get back on the road.

But as I was doing my brake lines I ran into a juction at the rear

above axel. It has two lines going into it or out of it :brows:

There is this thing hanging from it. It spins around but for what?

What is it and is it needed?

I've been reading some of the write up's but it is kind of losing

me :ack:

I read that some of you have change it but to what? :wall:

 

thanks for reading

[Akula69]

Well, I believe that you are talking about the rear proportioning valve (or prop valve). There are two such valves on the truck...one up by the master cylinder, and one above the axle in the rear. And, although mine doesn't, the "spinning" thing you mention is probably the load sensor that creates additional braking power if the truck is loaded.

 

You might want to check this thread for info, and follow the links in it as well for additional info and drawings. It mainly deals with changing out the booster and master cylinder, but also talks about the prop valve and it's function.

 

http://comancheclub.com/forums/viewtopic.php?f=2&t=12473&start=0&st=0&sk=t&sd=a

 

HTH :cheers:

[Eagle]

Well, I believe that you are talking about the rear proportioning valve (or prop valve). There are two such valves on the truck...one up by the master cylinder, and one above the axle in the rear. And, although mine doesn't, the "spinning" thing you mention is probably the load sensor that creates additional braking power if the truck is loaded.

I agree the thing in question is the rear proportioning valve, but it is incorrect to suggest that there are two in an MJ. The front metering block in the MJ provides NO proportioning, all it does is send brake fluid out through the different hard lines. That's why I refer to it as a "metering block" rather than as a "combination valve." The one in an XJ (Cherokee) does include a proportioning function, and is therefore called a "combination valve." The one in the MJ is completely different, even though the shape of the aluminum chunk looks the same on the outside.

 

If you have one that's just hanging, that means the actuating rod isn't connected to the differential, and that means the proportioning valve isn't adjusting for the height of the bed. It also means you have no idea if the proportioning valve is allowing full braking to the rear wheels, or NO braking to the rear wheels, or somewhere in between.

 

It's in situations like this (or, as in mine, when the thing blows out) that I advocate just removing it and running a single line to the rear axle from the front. The plus to that is you always know you have full rear brakes. The negative is that full rear brakes may cause the rears to lock prematurely in a panic situation, so if you go that route -- try the brakes out in some hard stops, and if the rear end gets too squirrely for your comfort level, either replace the rear wheel cylinders with smaller diameter cylinders, or put a Wilwood adjustable proportioning valve into the line to the rear.

[Akula69]

Well, I believe that you are talking about the rear proportioning valve (or prop valve). There are two such valves on the truck...one up by the master cylinder, and one above the axle in the rear. And, although mine doesn't, the "spinning" thing you mention is probably the load sensor that creates additional braking power if the truck is loaded.

I agree the thing in question is the rear proportioning valve, but it is incorrect to suggest that there are two in an MJ. The front metering block in the MJ provides NO proportioning, all it does is send brake fluid out through the different hard lines. That's why I refer to it as a "metering block" rather than as a "combination valve." The one in an XJ (Cherokee) does include a proportioning function, and is therefore called a "combination valve." The one in the MJ is completely different, even though the shape of the aluminum chunk looks the same on the outside.

 

If you have one that's just hanging, that means the actuating rod isn't connected to the differential, and that means the proportioning valve isn't adjusting for the height of the bed. It also means you have no idea if the proportioning valve is allowing full braking to the rear wheels, or NO braking to the rear wheels, or somewhere in between.

 

It's in situations like this (or, as in mine, when the thing blows out) that I advocate just removing it and running a single line to the rear axle from the front. The plus to that is you always know you have full rear brakes. The negative is that full rear brakes may cause the rears to lock prematurely in a panic situation, so if you go that route -- try the brakes out in some hard stops, and if the rear end gets too squirrely for your comfort level, either replace the rear wheel cylinders with smaller diameter cylinders, or put a Wilwood adjustable proportioning valve into the line to the rear.

 

 

Hmmm.. I don't disagree with you Eagle, but I've always seen both of them called "prop" valves up until now.

[CWLONGSHOT]

Prop is just short for proportioning valve. It "bais's" the brakes, USUALLY front to back. But in the case iof the MJ it works based on WEIGHT in the bed and only controls the REAR axle.

 

The front, althou it LOOKS like every other prop valve it is not, its just a junction to split the lines front to back.

 

CW

[Eagle]

Hmmm.. I don't disagree with you Eagle, but I've always seen both of them called "prop" valves up until now.

Oh, yeah, most people call it a proportioning valve, or a combination valve, because it looks like the XJ proportioning/combination valve. But in the MJ it doesn't serve any proportioning function, and in so it isn't a proportioning valve and it isn't a combination valve.

 

Most Internet "experts" also call the second brake line to the rear in an MJ a "return" line ... but it isn't. It's an emergency by-pass line. Don't believe everything you read. As a guy I used to work for often said, "Everything you read is true ... unless you have first-hand knowledge of the facts."