Another Proportioning valve thread

[Akula69]

I don't know if the PM messages are working in here yet, so I'll post this out for the masses to see what everybody else has been doing about the vacuum booster upgrade / rear prop valve.

 

First, I am (or so I believe) ready to swap my 89 vacuum booster and master cylinder with a 96 model. I was able to recover the entire assembly (including the proportioning valve and some brake line flare nuts) from an XJ. I also recovered the pedal assembly, switch and bracket, so I won't have any push rod issues.

 

I know that some folks have determined they want to dispose of the brake adjusting valve at the rear axle in the MJ, but I really don't feel it is necessary to do so, as mine is functional and not leaking. After searching a good bit I read the numerous posts about the vacuum booster swap and this is usually accompanied with a section on plugging an outlet on the MJ prop valve. I will not be installing the XJ prop valve because of it's problems with restricting the rear braking power, but I'm confused about how to deal with the lines. This thread indicates the outlet should be plugged and shows one line remaining to the back (which eliminates the bypass line):

 

http://comancheclub.com/forums/viewtopic.php?t=4968&highlight=prop+valve

 

Further research later I discover that others have indicated the bypass line should remain and be spliced into the main line going to the rear brakes.

 

Has anyone spliced them, and how does this work for you?

 

The only reason I can see to splice them is to overcome the fault in the MJ bypass valve boring (see thread above). Otherwise, the only way to not splice them is to remove the valve at the rear axle and only have one line.

 

Thanks in advance!

[Pete M]

That pic is of my setup and I did it like that back in '02 out of ignorance. I mistakenly thought that one line was a "return" and so plugged it when I deleted the prop valve.

 

Oh, and PMs are up and running gain. :cheers:

 

 

[jtdesigns]

The easiest for me to do was keep the MJ's prop valve. I just made two new lines using the flair nuts off the old lines. It was simple and works very well. Kelp the rear valve too.

[Eagle]

If you are keeping the rear proportioning valve, you MUST keep both lines to the rear. One is for normal (proportioned) braking. The other (the bypass) is to provide full power to the rear brakes if you lose the fronts.

 

If you keep the rear proportioning valve, DO NOT attempt to eliminate one of the lines or you will be creating a very unsafe condition.

[Pete M]

Hmmm. So what would happen in my 88 if the front brakes failed. I've eliminated the rear valve and plugged the bypass up front. Would the rear brakes still work if there was a problem with the fronts? Or would all the pressure be forced to the plugged port?

[Eagle]

Hmmm. So what would happen in my 88 if the front brakes failed. I've eliminated the rear valve and plugged the bypass up front. Would the rear brakes still work if there was a problem with the fronts? Or would all the pressure be forced to the plugged port?

Pete, the way you rigged your brakes is the way I rigged mine -- we have full power to the rear brakes all the time, so there is no need for the bypass port. If we lose the fronts, we have the rears.

 

The problem is plugging the bypass but leaving the normal (nose) line feeding through the rear height-sensing valve. When an MJ is empty, if the height sensing valve is adjusted correctly there is almost NO force going to the rear brakes. This is why the rear brakes never seem to wear out. The whole point of the bypass circuit is to get some braking in the rear if you lose the front brakes. Plug the bypass, and the only rear brakes you have if you lose the fronts will be the "proportioned" rears -- which effectively means virtually none.

[Pete M]

Ok, so when the fronts fail, and the bypass is engaged, but it does not close off the primary. Yes?

[Akula69]

What Pete said, yes, but also:

 

Depending on which option you choose, could not the design (manufacturing) flaw shown in the prop valve that was sliced apart effect your decision? See pic on second page here:

 

http://comancheclub.com/forums/viewtopic.php?t=4968&postdays=0&postorder=asc&highlight=prop+valve&start=15

 

If you choose to only leave the line to the bypass, and the valve body is not fully drilled it will not get fluid at all. To check it would require removal of the prop valve and the piston, then inserting a thin rod in there to see if it goes all the way through.

 

Is there a way to complete the drilling of this port without damaging the prop body? I suspect the neck on my press will not reach far enough into the throat of the port to complete the drilling, and a longer bit of that diameter would probably twist apart from the stress.

[mjtjnj]

fwiw I realized my prop valve wasn't working when I went through inspection and the skid pad showed that my rears weren't doing anything!

 

I got rid of the prop valve, ran one line to the back, and plugged the bypass. What I don't like about it now is, if I brake really hard, the rears lock up before the fronts, and causes me to skid a bit. I haven't figured out what to do about this yet, but I haven't been driving like we're taught to in NJ: I'm staying off people's backs! :nuts:

[Pete M]

fwiw I realized my prop valve wasn't working when I went through inspection and the skid pad showed that my rears weren't doing anything!

 

I got rid of the prop valve, ran one line to the back, and plugged the bypass. What I don't like about it now is, if I brake really hard, the rears lock up before the fronts, and causes me to skid a bit. I haven't figured out what to do about this yet, but I haven't been driving like we're taught to in NJ: I'm staying off people's backs! :nuts:

 

I would try swapping in the XJ prop valve up front. I believe the rears have a restrictor that reduces the power to them.

[jtdesigns]

You might want to give this a shot. http://www.shop.speedunlimited.com/Shop/Control/Product/fp/SFV/32401/vpid/4083676/vpcsid/0/rid/126429

 

 

That way you can manually adjust the rear bias, for extra weight in the rear, or what have you.

[Eagle]

You might want to give this a shot. http://www.shop.speedunlimited.com/Shop/Control/Product/fp/SFV/32401/vpid/4083676/vpcsid/0/rid/126429

 

 

That way you can manually adjust the rear bias, for extra weight in the rear, or what have you.

That's what I recommend, for those uncomfortable with rear brakes that really work. An alternate would be to use rear wheel cylinders with a smaller diameter pistol to reduce the rear braking somewhat.

 

FWIW, the Mopar Performance Catalog offers (or used to) an adjustable proportioning valve that I believe is the Wilwood. BLHTAZ may be able to get it for you for less than the price shown in JT's link.

[lower jeep]

ok you guys blocked off the return line with what? i asked my parts guy and he said that they offered nothing to block off a brake line so my question is what did you guys do to block off the return line?

thanks

[Eagle]

ok you guys blocked off the return line with what? i asked my parts guy and he said that they offered nothing to block off a brake line so my question is what did you guys do to block off the return line?

thanks

First, it isn't a "return" line. It's a bypass line. (Don't feel bad. I owned my MJ for at least 5 years before I realized that what everyone called the "return" line ... wasn't.

 

Second, the port in the metering block uses a straight machine thread. I don't recall if the size is 3/8" or 7/16" but you buy the shortest FINE (SAE) thread bolt you can in that size, then cut it shorter. You probably could just put some Loc-Tite on it and screw it in, allowing the end to seat against the raised cone in the block. That looks too home-brewed for my taste, so I cut it off so that the end of the shank just barely cleared the seat in the metering block with an O-ring on the bolt. Then I put a glob of black silicone RTV on the end and allowed that to set up. When I screwed the bolt in, I got (I hope) a redundant seal -- the blob of silicone on the seat, plus the O-ring under the bolt head. And it looks a lot neater to see just the hex head rather than seeing a half inch of thread sticking out.

[Andy in Pa]

ok you guys blocked off the return line with what? i asked my parts guy and he said that they offered nothing to block off a brake line so my question is what did you guys do to block off the return line?

thanks

First, it isn't a "return" line. It's a bypass line. (Don't feel bad. I owned my MJ for at least 5 years before I realized that what everyone called the "return" line ... wasn't.

 

Second, the port in the metering block uses a straight machine thread. I don't recall if the size is 3/8" or 7/16" but you buy the shortest FINE (SAE) thread bolt you can in that size, then cut it shorter. You probably could just put some Loc-Tite on it and screw it in, allowing the end to seat against the raised cone in the block. That looks too home-brewed for my taste, so I cut it off so that the end of the shank just barely cleared the seat in the metering block with an O-ring on the bolt. Then I put a glob of black silicone RTV on the end and allowed that to set up. When I screwed the bolt in, I got (I hope) a redundant seal -- the blob of silicone on the seat, plus the O-ring under the bolt head. And it looks a lot neater to see just the hex head rather than seeing a half inch of thread sticking out.

 

OR, you just go to the local Advance Auto Parts and buy a plug that is made to plug this hole. That is what I did. It is best to take the fitting with you so you can match it up, but the Advance store near me had exactly what I needed. I still put a dab of silicone on the threads, but this worked very well.

 

A.