Posessed hydraulics

[H-K]

It does not mater if it moves around or not. It is part of the closed system and is subject to Pascals Law. It will compress every time the same amount every time no matter where it is in the lines or calipers, ect resulting in the same loss of hydrolic pressure every time.

This is exactly what I was thinking. Thanks for explaining it better. What besides the Master cylinder would cause a sudden and only brief total drop in pressure. Keep in mind that I can find no leaks and after driving out to RS, running the trail, then 4 more hours of exploring Ophir, and driving back to Salt Lake resulted in no decrease in master cylinder fluid levels.

BTW, the first time the brakes were bled they were done by professionals who were sure it was a master cylinder problem.

thanks

PS anybody want to bleed these for me for a modest fee? I'm sick of fking with it.

 

[H-K]

It could possibly be air in the system though if there is so much that it causes the valve in the combo valve (I forget what it is called specifically) to slam shut preventing pressure from being lost to both ends when you lose pressure on one circuit. Maybe it is sticking closed sometimes and then popping open.

Do your frt and rear brakes actually work?

Do you mean the proportioning valve or the "valve" in the MC?

Yes, they work. I drove the rig for about 100 miles and the pedal was expectedly soft, but every once in a while the pedal would just drop to the floor.

One thing I'm just realizing is that although the problem is random, it doesn't occur at higher RPMs. Could the Brake booster be suffering from lack of vaccum at idle? It still doesn't explain why the problem is occasional/rare at idle. So, to clarify, I can't replicate the problem by just pushing the pedal at idle. It would only happen about 1 in 30 times or less. If the booster were failing, would it just have the same effect as killing the ignition? I'm thinking even if the booster failed, I would still have pressure....

 

[RockMonkey]

I have no idea what is going on with your brake system. Maybe it's time for an exorcist? I just wanted to interject that the YJ master cylinders seem to be fine with much larger brakes. I can lock up a set of 38s with my one-ton brakes and the stock master cylinder.

 

[H-K]

I have no idea what is going on with your brake system. Maybe it's time for an exorcist? I just wanted to interject that the YJ master cylinders seem to be fine with much larger brakes. I can lock up a set of 38s with my one-ton brakes and the stock master cylinder.

Know any cheap exorcists?


That's amazing that your stock MC handles those brakes. Maybe I should re-think the E350 MC and just focus on getting this prob fixed first.

Did you use the stock proportioning valve? If not, what type/brand did you use. Also, did you do rear discs?

 

[RockMonkey]

Master cylinder and proportioning valve are stock 93 YJ. I originally used stock One ton Ford front discs and stock 3/4 ton Ford rear drums. I could lock up both axles, until the rear axle seals failed and soaked the drums with gear oil. I have since upgraded the rear 60 to Chevy discs, and I can lock those too. I don't see any reason to swap out the YJ master cylinder, unless of course it is bad.

 

[chans]

Do you mean the proportioning valve or the "valve" in the MC?

Yes, they work. I drove the rig for about 100 miles and the pedal was expectedly soft, but every once in a while the pedal would just drop to the floor.

One thing I'm just realizing is that although the problem is random, it doesn't occur at higher RPMs. Could the Brake booster be suffering from lack of vaccum at idle? It still doesn't explain why the problem is occasional/rare at idle. So, to clarify, I can't replicate the problem by just pushing the pedal at idle. It would only happen about 1 in 30 times or less. If the booster were failing, would it just have the same effect as killing the ignition? I'm thinking even if the booster failed, I would still have pressure....

I had the booster go out in a car and all it does is make the pedal harder to press like older cars and trucks that don't have power brakes. It just helps aid your foot. I know someone that had air in the line and the brakes were fine for awhile then all of the sudden the pedal went to the floor but after that it kept going to the floor though.

 

[James K]

It does not mater if it moves around or not. It is part of the closed system and is subject to Pascals Law. It will compress every time the same amount every time no matter where it is in the lines or calipers, ect resulting in the same loss of hydrolic pressure every time.

I have a hard time believeing that its not air in the line causeing the problem especially if there is no leaks.



did the MC get bench bled?


will then I guess I'll go back to my corner.

 

[H-K]

I have a hard time believeing that its not air in the line causeing the problem especially if there is no leaks.



did the MC get bench bled?


will then I guess I'll go back to my corner.

I'm not ruling out air in the lines by any means...yet. I just really expect the symptom to be more consistent if it were air in the lines.

Although not done on a bench, I did "bench bleed" the MC. I installed it and bled it with the pedal, watching the bleed tubes through the windshield. I was very careful to keep tapping the MC body to free up trapped air. This took a long time before the bubbles finally stopped appearing.

I'm going to re-bleed the system this weekend and if it doesn't fix the problem, then maybe i can at least rule it out.

 

[crimsonride]

This may sound stupid but check your resivour, sounds to me like the MC isn't getting fluid.
Also, wasn't that the year that jeep put that little compressor motor on the brake systems? Maybe that could be something to do with it.

 

[H-K]

This may sound stupid but check your resivour, sounds to me like the MC isn't getting fluid.
Also, wasn't that the year that jeep put that little compressor motor on the brake systems? Maybe that could be something to do with it.

I'm not sure what you mean by checking the reservoir. Do you mean check to see if it has fluid? If so, I can assure you it does.

I'm not sure what the compressor motor thing is. Is it something that works with the booster?

thanks

 

[chevtech]

I'm not sure what you mean by checking the reservoir. Do you mean check to see if it has fluid? If so, I can assure you it does.

I'm not sure what the compressor motor thing is. Is it something that works with the booster?

thanks

Not to be a broken record but I have seen ABS accumulators do what you are describing repeatedly. I know you don't have working ABS now for sure but are you absolutely positive there is no left-over components from back when? They would have to be inline with the brake lines of course.

Sorry last time I will ask.

 

[mbryson]

This may sound stupid but check your resivour, sounds to me like the MC isn't getting fluid.
Also, wasn't that the year that jeep put that little compressor motor on the brake systems? Maybe that could be something to do with it.

I THINK that's with the ABS systems.

 

[H-K]

Not to be a broken record but I have seen ABS accumulators do what you are describing repeatedly. I know you don't have working ABS now for sure but are you absolutely positive there is no left-over components from back when? They would have to be inline with the brake lines of course.

Sorry last time I will ask.

I've never worked on anything ABS so I don't know what to look for. i would guess that it would be connected at or near the proportioning valve? I'll look tonight for any vestigal ABS stuff that I may have not noticed before.

 

[chevtech]

I've never worked on anything ABS so I don't know what to look for. i would guess that it would be connected at or near the proportioning valve? I'll look tonight for any vestigal ABS stuff that I may have not noticed before.

I don't know where Jeep put there ABS stuff but if you just follow the brake lines down from the master cylinder the only thing between the master cylinder and the wheels should be the combo valve (otherwise known as a proportioning valve) if there is anything you're probably looking at the ABS unit.

They are located in all different areas on different vehicles.

 

[H-K]

I don't know where Jeep put there ABS stuff but if you just follow the brake lines down from the master cylinder the only thing between the master cylinder and the wheels should be the combo valve (otherwise known as a proportioning valve) if there is anything you're probably looking at the ABS unit.

They are located in all different areas on different vehicles.

so it's not necessarily between the MC and the valve? Would it still be before the lines become too distant from one another or are there separate units for front and rear that could be located where they split off from L to R? could there be separate units for each wheel? I always assumed that ABS would manage pressure near the Proportioning valve, but then like I said, I've never worked on or looked at ABS components.

thanks

 

[mbryson]

so it's not necessarily between the MC and the valve? Would it still be before the lines become too distant from one another or are there separate units for front and rear that could be located where they split off from L to R? could there be separate units for each wheel? I always assumed that ABS would manage pressure near the Proportioning valve, but then like I said, I've never worked on or looked at ABS components.

thanks

To bleed my '91 XJ with ABS, you need to bleed the master cylinder, then the pump, then the lines as you would in a normal system. You just bleed by cracking open the fitting on the master cyl and the ABS pump, then you move to the traditional bleeder screws at each wheel. The only pain is that the ABS uses so much brake fluid (probably a good thing, ultimately) that it's difficult to keep the master full. An assistant is very handy to keep fluid in the master while you have another pumping the brake pedal. Kind of a pain, but not too serious. Your YJ, may or may not be similar.

 

[chevtech]

so it's not necessarily between the MC and the valve? Would it still be before the lines become too distant from one another or are there separate units for front and rear that could be located where they split off from L to R? could there be separate units for each wheel? I always assumed that ABS would manage pressure near the Proportioning valve, but then like I said, I've never worked on or looked at ABS components.

thanks

No there won't be one unit for each wheel and yes they are usually between the master cylinder and the combination valve. Sometimes though they have them underneath and run the lines all the way underneath and than back forward. Sorry I don't know for sure where they are on a YJ. Maybe someone else will know.

If you have a bad accumulator, bleeding it probably won't help (but would still be worth a try) but you can just bypass it since you wouldn't be using it anyway (assuming you even have ABS).

 

[H-K]

No there won't be one unit for each wheel and yes they are usually between the master cylinder and the combination valve. Sometimes though they have them underneath and run the lines all the way underneath and than back forward. Sorry I don't know for sure where they are on a YJ. Maybe someone else will know.

If you have a bad accumulator, bleeding it probably won't help (but would still be worth a try) but you can just bypass it since you wouldn't be using it anyway (assuming you even have ABS).

Okay, no ABS then. There's nothing between the MC and the valve.
thanks anyway

 

[H-K]

Reviving this thread to thank everybody for their help and input and to also explain what the cause was to those who are curious.

Here's what was happening: The right front caliper banjo fitting was coming in to contact with the knuckle when turning sharply. The knuckle was putting lateral force on the fitting, causing it to leak. There are adjustable stops for the knuckle that were not set to prevent this.

To remedy the situation, I extended the stop (bolt) to it's max, but it still was not enough to prevent contact. I couldn't find a suitable longer replacement in my garage, so I welded a nut to the head of the bolt and added a big booger to the tip. This took care of it.

For whatever reason I never found any fluid that would clue me in to this conclusion. I assume it dripped directly off the fitting and thus did not leave any runny traces on any components. I also never found any puddles of fluid on my tracks for some reason.

Red Herring: I had falsely associated the pressure drop symptom with slow speeds or low RPM and never noticed that the one common clue was that it happened only when making turns where the steering was maxed out to one side. Coincidentally, you only make such sharp turns at low speeds and if in gear, your RPMs would be very low.

Note that neglecting to protect the fitting is the shop's fault not mine.

Thanks again.