proportioning/combination valve in 89 XJ

Hi Guys,

I know this topic has been discussed but could you XJ gurus tell if what I plan on doing is correct?

The prop/combo valve on my brake system is causing low volume to the rear brakes...been running it like this for a while and my buddy helps me through inspection on the dyno using the E-brake to slow down. Except for the valve, all components on the brake system are new.

I have another valve on the bench that I pulled the frozen piston out of and want to use this valve in conjunction with an adjustable Wilwood prop valve to clamp the volume going to the rear brakes.

When I first started thinking of using the adjustable valve I assumed it was a replacement for the original but appears that there is only 1 input and 1 output.

Thanks for all input,
Jay in MA

CHW had the same problem on his '89 -- no rear brakes with the big drums on a Dana 44, no matter what he did. He finally took out the factory combo valve, replaced all the hard lines with new, and used a Wilwood adjustable proportionaing valve. He said he now has all the rear brakes he needs with the Wilwood dialed in somewhere in the middle.

I'd say go with the Wilwood, but skip the factory unit. Plumb the front brakes direct (no valve) and put the Wilwood into the line feeding the rear circuit. The purpose of the proportioning valve is to reduce the braking in the rear enough so the rears won't lock up before the fronts, causing the vehicle to switch ends in a panic stop. There is no need to reduce braking to the front.

Thanks Eagle...I hadn't thought of that option. It might be a cleaner way of doing it.

I'll report back when I finish the project. Both daughters are driving this rig now and I would prefer a bit more rear braking than it is presently getting.

Thanks again,
Jay in MA

The factory combo valve serves one other function, and that's to trigger a warning light if either the front or rear brake circuit fails. Scrap the combo valve and there won't be any warning light.

This may not be legal, but as a practical matter I see it as a non-issue. At least on my 88s, you can't see the warning light in full daylight unless you really look for it. When the rear brake line on the MJ popped recently, I felt it immediately, and the peddle is noticeably lower than it was. Same was true when the rear brakes let go on the XJ a couple of years back. If your daughters are driving the vehicle you might want that light, but IMHO it's useless and if I had a choice, I'd rather have good brakes rather than a light that I don't need to tell me when the brakes have failed.

Don't mean to hijack the thread but where do you mount the Wilwood pop valve? Just plumb it into the inside?
Thanks
Keith

It can go anywhere in the hard line from the master cylinder to the rear axle. CHW put it under the hood near the MC. The only reason to put it under the florr and cuta hole for the adjuster knob into the cabin is for a racer who needs to adjust the braking on the fly to compensate for tire wear or track conditions. The hole in the floor is not a good idea in a 4x4 that fords streams.

Thanks again Eagle.

I'm not concerned with the light either for the same reasons you stated.

I was also thinking that CHW's way of doing it might be better as the gutted out prop valve might blow out the "dummy" switch threads with the full pressure being allowed to pass through the area previously occupied by the piston...any thoughts on that??

Jay in MA

Jay Welch said:

Thanks again Eagle.

I'm not concerned with the light either for the same reasons you stated.

I was also thinking that CHW's way of doing it might be better as the gutted out prop valve might blow out the "dummy" switch threads with the full pressure being allowed to pass through the area previously occupied by the piston...any thoughts on that??

Jay in MA

Click to expand...

I'm not sure I understand how you were thinking of modifying the combo valve. I fact, I'm sure I don't understand, since I've never had one apart and don't know how it works. I don't think there would be a problem with blowing out the switch, but if you remove the piston does that cross-connect the front and rear circuits? If so, that defeats the purpose of a dual-circuit master cylinder and makes it possible to lose ALL brakes if a line on any wheel lets go.

What did you do about the residual pressure for the drums? Did you use a valve in the rear brake lines?
Keith

Well... originally I was going to remove the piston that activates the light as I understand that not only does it set the light in the case of unequal pressure in a fluid loss condition but it also proportions the fluid to the rear. I suspect this is the culprit in the low volume to the rear brake system.

Then when you mentioned CHW's solution it got me to thinking it would be much cleaner to do it his way and as you suggested (and I think you are correct) the dual system might be negated by removing that piston.

I ordered the Wilwood prop valve from Summit today and will eliminate the OEM prop valve, reroute the hard lines direct from MC to front rubber lines and add the new adjustable prop valve between MC and rear lines. For anyone else that is interested in this upgrade the cost was $48.44 for new valve and handling. PArt number was WIL-260-2220.

I'll let y'all know how it works when I get it done.

Thanks for all of the feedback.
Jay in MA

keith haw said:

What did you do about the residual pressure for the drums? Did you use a valve in the rear brake lines?
Keith

Click to expand...

We had hydraulic drum brakes for almost a century before some idiot decided we needed "residual pressure" for the drums. If you keep the brakes properly adjusted I can't think of any reason why you would need or want residual pressure in the system when you are not applying the brakes.