brake failure due to large leak

01 XJ, 4" lift, 32" tires, WJ front brake conversion, 98 GC rear disc brake conversion.
I drove through some accident debris, something bounced off the roadway & put a hole in the passenger side front brake hose; pedal went to the floor "no brakes"
Luckily I had room to coast dropping the transmission to lower gears until the emergency brake could stop the jeep. I was towed off the highway, purchased & installed a new hose, bled the brakes & was on my way.

My question is with the divided master cylinder design of the jeep, shouldn't the rear brakes have still worked? Isn't the point of a divided master cylinder to prevent loss of all braking incase of just the thing that happens to me?
Thanks for any answers in advance!

Yes, the rear brakes should have worked. They probably are not doing their job now. Did you do anything with the proportioning valve?

Hypoid said:

Yes, the rear brakes should have worked. They probably are not doing their job now. Did you do anything with the proportioning valve?

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Yes, I performed the prop valve mod that is recommended with the disc brake swap, swapping out the internals (the spring &"piston) from a the prop valve of a rear disc brake equipped grand Cherokee.
http://go.jeep-xj.info/HowtoPvalve.htm

I think I found the answer, if I'm reading this correctly the prop valve mod removes the front/rear isolation,
Can some one Please confirm this for me, if correct I'll UnDo the mod & put the prop valve back to OEM status as I'd rather have the Front/Rear isolation than the slight increase in fluid to thevrear under hard braking.

Quote from the article
"the other thing that is often done is modifying the proportioning valve to get more fluid to the rear. Remember though that this removes the isolation between front and rear so a failure at either end would leave you without enough pressure to the other! For that reason I DONT RECOMEND THIS MODIFICATION! It is only included to show what is done by some but you do so at your own risk"!

If you gutted the proportioning valve, then yes, it is now merely a distribution block. Put the shuttle valve & spring back in there!

yossarian19 said:

If you gutted the proportioning valve, then yes, it is now merely a distribution block. Put the shuttle valve & spring back in there!

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I didn't just pull it, I swapped the internals from a rear disc brake ZJ into the XJ prop valve.

I know on my old ('89) XJ that the front and rear were completely separate with two sections in the master cylinder reservoir. My new (99) and I assume your 01 only have one master cylinder reservoir. Even with a perfectly functioning proportioning valve, I don't see how you could still use the rear brakes after losing a hose as the fluid would all leak out quickly.

iluv83vettes said:

I know on my old ('89) XJ that the front and rear were completely separate with two sections in the master cylinder reservoir. My new (99) and I assume your 01 only have one master cylinder reservoir. Even with a perfectly functioning proportioning valve, I don't see how you could still use the rear brakes after losing a hose as the fluid would all leak out quickly.

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I can "recently" confirm this. I lost a front brake line with a stock proportioning valve, and proceeded to loose all fluid in one pump. My rears did not have enough fluid to function, and I had to use vise grips to pinch the line to get off the trail.

xeroOTG said:

I can "recently" confirm this. I lost a front brake line with a stock proportioning valve, and proceeded to loose all fluid in one pump. My rears did not have enough fluid to function, and I had to use vise grips to pinch the line to get off the trail.

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Wow, I thought all modern vehicles used a split reservoir as a safety measure; I Don't like this at all.

garr said:

Wow, I thought all modern vehicles used a split reservoir as a safety measure; I Don't like this at all.

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I have to wonder myself, my 75 CJ5 has a split reservoir. Something either changed technology-wise and you and I have a failed valve, or its no longer a requirement and they stopped using dual reservoirs.

The reservoir is partially separated into two sumps about half way down even though it only has one cap to fill it. The sumps are to hopefully avoid the situation you had of the whole system loosing pressure due to fluid loss and keeping at least enough fluid in the functioning side sump for that half to still be useable. As to why you lost all braking ability can be a number of things, proportioning valve or master cylinder malfunction, air in the rear lines or simply the master cylinder can't move enough fluid to the rear brakes to clamp them completely. You mentioned the WJ and GC brake upgrades, are you still on the original XJ master cylinder?

civic50 said:

The reservoir is partially separated into two sumps about half way down even though it only has one cap to fill it. The sumps are to hopefully avoid the situation you had of the whole system loosing pressure due to fluid loss and keeping at least enough fluid in the functioning side sump for that half to still be useable. As to why you lost all braking ability can be a number of things, proportioning valve or master cylinder malfunction, air in the rear lines or simply the master cylinder can't move enough fluid to the rear brakes to clamp them completely. You mentioned the WJ and GC brake upgrades, are you still on the original XJ master cylinder?

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Yes, 2001 MC, I Read that the WJ MC would provide a slight increase in performance over the late model XJ unit but its not worth the effort or $$.

You may be correct about the fluid volume causing my issue.
Any input is greatly appreciated
Thanks.

The WJ master cylinder is the same bore as the XJ unit, 1". It will not provide any additional clamping force.
If you have an older XJ with a single diaphragm brake booster, a WJ unit is more readily swapped in than a newer XJ unit and it is a dual diaphragm booster (which will increase line pressure & improve brakes)

FWIW, I had stock front brakes and ZJ rear discs at one time. Suffering a brake flare failure (hey, it was my first time doing it...) leaving a parking lot, I lost all brakes as I rolled down the icy road toward a stop sign. Thankfully I was only doing 5 mph and nobody was around...

On the job, I have also seen age / maintenance related issues with Volvo distribution blocks, causing the "brake failure" light to come on and stay on on the dash. Not a safety issue, but still not as designed...

My point in mentioning my experiences w/ the Jeep and Volvos is that i think the brake failure is as simple as safety equipment that routinely does not perform as designed. Your master cylinder is large enough, your prop valve is (probably) fine - you just don't want to ever count on this stuff working as it's intended.

I was crawling around under my Jeep today looking things over & noticed the long solid brake line going to the rear has seen better days, NYS`s liberal use of salt on the roads has taken its toll. next week I'll replace it & the rest of the hard lines + soft lines. While I'm at it I'll swap out the prop valve (I picked up a spare off a 98 GC with rear disc @ the junk yard for $10 today) & hope it is the problem.

Before you get going on the brake lines, let me recommend Ni-Cop brake lines. It's a nickel & copper alloy, used by Volvo (safety kings) from the factory. It doesn't rust, it has a burst pressure much higher than it needs to be (again, used as OEM equipment on Volvo and probably others) but here's the reason I love it most: the stuff is stupid-easy to bend, flare & route. SO much easier than steel. I'll never buy another roll of steel line: ni-cop is *that* much easier.

yossarian19 said:

Before you get going on the brake lines, let me recommend Ni-Cop brake lines. It's a nickel & copper alloy, used by Volvo (safety kings) from the factory. It doesn't rust, it has a burst pressure much higher than it needs to be (again, used as OEM equipment on Volvo and probably others) but here's the reason I love it most: the stuff is stupid-easy to bend, flare & route. SO much easier than steel. I'll never buy another roll of steel line: ni-cop is *that* much easier.

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Thanks, I picked up a 30' roll of Ni-Cap & fittings a while back, it is much easier than steel & about 1000 times easier than stainless to work with.
The Plan is to take out each line individually & match it as best as I can, One at a time.