Brake Combo Valve

[The Anti-Chrysler]

I just put a Rusty's 4" lift on the J-10, and got new stainless brake lines. When I was going to bleed the front brakes, I had someone press the brake pedal while I tried to grab that piston at the front of the combo valve - and it is frozen. :rolleyes: :eek:

It was my understanding that this should be held out while bleeding the front brakes. Is this going to screw up my braking since it is frozen? Should I change the combo valve?? I wonder if by being stuck it is limiting my front braking. Any comments? Thanks. tongue.gif

[seabell]

Jason, does the pin push in? Many combination valves work this way. Not sure about an '82 J-10, but my '79 Cherokees are both the push-in type.

[andy d]

uhhmn, i just replaced a front caliper and did nothing with the valve. just make sure to keep the reservoirs full in the mc.

[The Anti-Chrysler]

Charlie,

I don't think the pin moves period. It definitely didn't go anywhere, in or out, when someone depressed the brake pedal while I was under there. It was my understanding that it should be held out, so that when you're bleeding the fronts, it can't proportion and leave an air pocket. How can it be held when it goes in on your type?? I am used to later model GM types where the pin extends out.

In either case I probably have to replace it because the pin doesn't move at all, whether bleeding or not. The brakes are OK, but no matter how much I bleed them, and get no air, they always work better when pumped once or twice first.

[bvibert]

On the kind you are supposed to hold in you are supposed to use a tool that bolts to the front bolt of the prop valve. It is essentially a L shaped peice of metal with a slot for the bolt. See this link:

http://www.ifsja.org/tech/brakes/brakevalvetool.shtml

Either way it sounds like yours is shot...

[The Anti-Chrysler]

Yeah, I guess it's shot, I was just wondering how much it was screwing up my braking by having the valve stuck - guess it depends where it's stuck.

But I'M SURE that all of the lines will come right off nice and easy so I can change it out... HAHA!!!!

[bvibert]

Sure they will ;) If it makes you feel any better I totally forgot about that little pin when I changed my calipers (remembered the next day, but was way too lazy to bleed again) and my brakes seem fine...

[okidoc]

I just did a Rusty's lift as well. The pin needed to be pulled out on my 87. Opening the front system did not alter the pin, but after I opened the rear to replace the brake hose I had to reset it. The link above from bvibert worked for me. I had the 'W' type.

Good luck.

[The Anti-Chrysler]

As I understand it, the valve pin needs to be held while bleeding the front brakes. The valve is what proportions the front to rear brake pressure ratio. If the pin isn't held while you're trying to bleed the system, I think it will trap air in there. I think it will see the open lines in the front as a pressure imbalance, and try to compensate by braking the rears. That's why it has to be held. This is my interpretation of the theory on the unit. I suppose I could be wrong in that the valve regulates the rear brakes, and not the front, so it would be reverse what I described above. I think I'm going cross eyed!! haha

The pin never "resets" I don't think, or stays in one spot. It should move anytime the brakes are depressed. The front brakes always will have a bigger percent of braking duty.

[seabell]

Actually, Jason, I believe that the valve with the external pin is the threshold valve, and it's function is to prevent the front brakes from coming on until the rear shoes are in contact with the drums. Remember, the front pads always skim the rotors (there's no return spring), but the rear shoes have springs which retract them well off the drum.
If you are not messing with the rear brakes, or have already bled them, I can't see why you need to activate (open) this valve in order to bleed the front. I have never bothered, and my brakes bleed fine.

[seabell]

Look at this:
http://www.hunter.com/pub/undercar/2690T/2690C.gif

More pretty pitures here:
http://www.hunter.com/pub/undercar/2690T/2690T.HTM

[The Anti-Chrysler]

NICE!!! tongue.gif :D

It looks like the two ports at the pin (at left) are the front brakes from what I remember.

[The Anti-Chrysler]

That looks like the kind you hold OUT. I can see where if it wasn't, it would trap air when you were trying to bleed the fronts. The valve space would always have an air pocket.

[The Anti-Chrysler]

Charlie, so I actually need to hold the pin (out or in, depending on type) when I'm bleeding the rears? I think I saw something about it sensing pressure from the shoes contacting the drums, as you mentioned. I would just like to know how it operates, and not just what to do to bleed it!! Makes me feel more educated. heehee. Thanks!

[seabell]

From the same site (Hunter):

QUOTE

Because of static weight distribution and rear-to-front weight transfer during braking, front brakes have more stopping power than rear brakes. The faster the stop, the greater the weight transfer. As a result, most systems use controls to regulate front and rear braking action. Two valves are common:

Metering Valve-Located in the front brake circuit, the metering valve remains closed until the front brakes reach a specific pressure. The valve's action prevents the front discs from applying until the rear shoes contact the drums, ensuring that front and rear brakes are applied at the same time.

ENDQUOTE

Assuming your front brakes are working, and the valve really IS stuck, this will mean that your front brakes will come on slightly before the rear. I still don't see hy you should need to manually open this valve UNLESS you are bleeding using the gravity method. If you bleed by slowly pumping, then the valve will open by itself after about 10 psi is applied.

[The Anti-Chrysler]

Enough said. Much abliged! smile.gif :D

[seabell]

Good. You're welcome!

Can I go home now? smile.gif

This cr*ppy keyboar is riving me nuts, an it's 5:30 here...

[The Anti-Chrysler]

Pat yourself on the back for me!! :D Yes, you may go now. tongue.gif

Didn't mean for you to have to ring in O/T for posting!! smile.gif

[LaJ10]

another quote from the same site.

Located in the rear brake circuit, the proportioning valve limits pressure to the rear drum brakes so they will not lock during hard braking. Both the metering and the proportioning valves are often combined with a brake warning light switch in one assembly called a combination valve