Brake proportioning valve???

I completely rebuilt the brakes on my 1971 CJ5 V6. New master cylinder, all new wheel cylinders, new brake shoes, new drums, and new fluid. Once it was back together the brakes just wouldn't stop the vehicle properly. The pedal wasn't mushy, not a lot of play, but it just seemed like I had to press on the pedal extremely hard to get the brakes to engage. The wheels would not lock up no matter how hard I pressed.

After working on it for a few days I finally took it to Brake Masters, because I had run out of ideas. The only think that they can find wrong is that there seems to be a lack of fluid pressure to the front wheel cylinders. They said that there was probably either something wrong with the new master cylinder (either defective or the wrong one), or it might be the proportioning valve. I advised them that I did not think that I had the wrong master cylinder, because there is only one listed for my application, and it appeared identical to the one that was replaced. The master cylinder was new, and not rebuilt, so I don't think it was defective. I advised them that the old brake fluid was extremely dirty, and that the brake lights were getting stuck in the "on" position after the rebuild, so I thought that the proportioning valve may be clogged. They are in the process of checking that now.

Am I on the right track?

Can a brake proportioning valve be cleaned out, or does it have to be replaced?

If I need a new proportioning valve, where can I get one?

 

A proportioning valve can go bad. I don't remember if yours should have one or not.
The brake light switch most likely is clogged with debris which is why the lights are staying on. Common problem. On a '71 and older they are hydraulically operated by brake pressure from the master cylinder.
Just because a master cylinder is new and not rebuilt doesn't mean it can't be bad. Been caught by this many times. Did you get the lines to the master cylinder mixed up possibly?
Also, checking the master cylinder should be easy. Crack each line to the master cylinder slightly and individually while holding down the pedal and see if you get fluid. If you don't get anything from one circuit then there is the culprit. You can do the same thing before and after the the proportioning valve (if equipped) to check it also.

 

Did you adjust the brakes ?

 

Brakes are adjusted. it's just that there doesn't seem to be enough hydraulic pressure to the brakes. Brake Masters said that there was weak stream when they bled the front brakes, but that the rear had a good stream. There seems to be plenty of pressure at the master cylinder. Maybe a clogged brake line?

 

Not sure if it's a proportioning valve or simply a distribution block but, my '71 has "one." I'll get a picture loaded soon. It mounts on the frame forward of the M/C.

 

Pretty sure there were no brake proportioning valves in CJs until 1974. I know there were none in '72 and they were present for '74. The CJ valve for drum brakes has a proportioning function and a pressure differential switch (brake failure warning switch). Take a look here http://oljeep.com/gw/74_tsm/9-BrakesWheels.pdf at fig 9-11 and surrounding text for the earliest combination valve in a CJ, AFAIK.

 

X2 on being distribution block

 

You are right Tim. The brake diagram in the FSM show a proportioning valve, but indicates that it is only for early models. Mine's a late model (1971). I took the diagram to Brake Masters, and showed them on the Jeep that there was no valve. I explained that since the rear brakes had a good flow of brake fluid, but both of the front brakes had a restricted flow, that there is probably blockage between the 2 front tees, 1 & 2 on the diagram. They are supposed to check it tomorrow. This is the first time that I have done a brake overhaul, and I'm explaining stuff to "Brake Masters"??? Scary!

 

Isn't there 2 different size wheel cylinders front and back ? Can reversal of those do that ?

 

10-4 they are different sizes. The rear are smaller 13/16", and the front are 1". They are installed correctly.

 

Just testing you

 

Some master cylinders have a stepped bore so apply different pressures to the front and rear. I reiterate, is it possible you reversed the lines going from the master cylinder? Others have been caught by this on this board and displayed similar symptoms.

 

The lines aren't reversed. They are factory, and can't reach to the wrong outlet. The old brake fluid had rust in it from the old master cylinder. I'm still thinking there's a clog somewhere. Hopefully I'll find out today.

 

If I had taken my Jeep to a shop which claims to be an expert as the their name implies (Brake Masters ?) and then had to explain how to trouble shoot the problem, I guarantee it would be back home in my shop NOW!

You yourself can easily test the brake lines for blockage. From viewing your comments and questions, you appear to understand the system. With the help and guidance of those here on this site, I feel you are more than capable of completing this repair.

Ed

 

I'm picking it up in the morning. They called me and told me that their "expert" couldn't figure it out. The only consolation if that there is no charge, as it should be. I'll check it out next week. I think that I'll figure it out, and as you said I can always get great help from the true experts here on this forum. Thanks to all who have assisted so far. I'll further info as I work through the problem.

 

The replacement master cylinder has been properly bench bled right? During this process you should be able to see both circuits output. You mentioned a 'weak' stream from the front port on that MC. I'd look at making sure it has been completely bench bled.

Don

 

I used a master cylinder bleed kit, and both tubes seemed to prime the mater cylinder, so I don't think that is the problem, but I'll re-check that if I can't find any blockage in the front lines.

 

I have the exact same symptoms on my 69'... replaced the MC as well to no avail. To do anything close to an abrupt stop, it's like doing a leg press on the brake pedal! haha

 

As you systematically brake down the system looking for blockage, if you find that the differential switch has failed these people may have the replacement. http://inlinetube.com/Prop Valves/pro_valves.htm Don't know anything about them (haven't used them) but are the only ones hat I could find which seem to have the replacement parts. Also, their tech support may help you diagnose the problem. Give them a call. The differential switch for your Jeep appears to be the same as a Mopar switch and is their listing p/n BLK250.

I may have missed this but, what was the reason for the brake rebuild in the first place (Pease of mind, weak brakes, pulling to one side or the other, etc.)? The original symptoms may help us help you figure this out.

 

The reason for the rebuild was that I had a leaking wheel cylinder, and rust in the Master cylinder.