TJ Brake Failure—Uncommon Symptoms

[ApexOffRoad]

Hey Everybody,

I have a modified 99 Jeep Tj. Basically everything has been replaced over the past 6-7 years. New 6 cyl. high performance motor, new automatic transmission, Atlas transfer case, Dynatrac axles. ARB lockers, Rubicon Express 6" long arm lift and so on. Drive shafts, steering and nearly everything else is also new. The only components ignored were the heart of the braking system with the original master cylinder being retained. New axles did come with upgraded disc brakes from Dynatrac. We attempted to cross the Rubicon trail this last weekend with a group. Believe it or not this was the first real off road outing for this Jeep (long story). To date it has only seen regular around town and freeway driving with no significant issues to report. At the last minute my other jeep developed an issue so I hitched up our "Chaser" trailer to the TJ (1600-1800 pounds fully loaded) and off we went.

Go ahead with the comments, I deserve them. The TJ isn't really set up to handle the trailer. The suspension is too soft for that amount of tongue weight and for a first time trip of this nature the Rubicon probably wasn't the smartest choice. We made it through relatively unscathed although pulling that trailer chunked the crap out of my tires with the excessive wheel spin needed to get up obstacles with a trailer in tow.

There was a significant issue uncovered on this trip that I need help with. Under load and especially when both axles were locked the brakes would fade on the UPHILL portions of the trail. No use of the brakes on the way up but as soon as I reached the top there was absolutely no pedal. Had to shift to neutral on nearly every occasion and pump the brakes to regain pedal pressure. When I say no brakes, I mean absolutely no resistance all the way to the floor. The problem got worse day by day. After the first day we bled the lines but there was no indication of air. As you know the Rubicon is a series of uphill and downhill sections. Even though the uphill braking issue seemed to get progressively worse the downhill portions seemed to function okay. When engaged in 4 wheel low the transmission wants to overpower the brakes, you can still adequately stop but shifting to neutral is the only way to stop normally. When you add lockers the brakes are no match for the forward motion that is being produced. Of course I switched off lockers on the downhill portions and out of uncertainty I let gravity bring me down the hill for much of the trail. The brakes seemed to resume normal operation with the transmission in neutral, even with the weight of the trailer behind. As we went further there were a few issues of the "soft pedal" beginning to happen on the flat sections of the trail and once while in reverse. My mind immediately goes to the Master Cylinder as the root of the problem but one of my mechanic friends questioned the ABS system, another friend said it sounds like I was loosing vacuum because of the significant rpms needed to get up the hills. Each time I shifted to neutral and pumped the brakes repeatedly the pressure and braking came back. I would appreciate ay insight anyone could offer before I tear into this issue. After this experience, I'm prepared to do whatever to fix the problem. Losing brakes on the trail with a trailer in tow was not fun.

Thanks,

Gary

 

[Steel City 06]

If you’re losing vacuum or the booster is bad, the brake pedal will get excessively stiff, not soft, just like after the engine has been off for a while. Perhaps there is something allowing air to get in the lines? If there’s a bad connection somewhere, and air is entering the lines, the pedal will get extremely soft and have to be pumped.

I would start with an aggressive brake bleed, followed by a basic leakdown test. To test, compress the brake pedal as hard as you can with a bar clamp spreader or similar tool, and leave it. If it loses pressure over time, there is likely a leak somewhere.

You may also be able to vacuum test the brake fluid system with a vacuum test kit. It’s possible that a leak could act like a check valve and let air in, but not fluid out.

Has the brake fluid level remained roughly constant?

 

[toximus]

1) When you said everything was replaced what did you replace as far as brakes go?

2) Are the rotors getting hot and/or pads smoking?

3) Do you have this issue when the trailer isn't hooked up?

4) What size tire do you have?

 

[ApexOffRoad]

Thanks guys, the fluid is stable and not leaking and does not appear to have boiled or gotten excessively hot. I did add a very small amount at the beginning of the trail after the first loss of pedal incident. I have not had this loss of pedal pressure issue without the trailer but then again I have never really "wheeled" the jeep this hard until now. Rotors and pads were not hot or smoking. Most of the issue was after a climb and higher RPM's, not excessive braking. When slowly creeping down the hill in neutral the brakes gave me no trouble at all and although they got a little spongy (normal) on the longer descents they quickly returned to normal when allowed to cool. They never smoked or smelled excessively hot.

Brake plumbing system and components are largely stock until you get to the steel braided extensions at the axles. The stock system with original Dana 44 rear axle had drums in the rear. The replacement rear Dynatrac 60 has disc brakes (I believe they were from Ford Explorer if my memory serves me). The Dynatrac 44 in the front also came complete with new disc brakes. Dynatrac was very good about helping me size the axles and providing information I would need to support the swap. They never mentioned brake system upgrades to support the changes we we're making and again, without the trailer or previous experience running a difficult trail I have not witnessed this problem.

I'm currently running 35" tires. When working with Dynatrac to get the axles and gearing right I specified that the largest I would want to go is 37"s.

I will try the leak down test and attempt the vacuum test...good starting point...thanks.

 

[CharlesHS]

I seem to remember (I may be wrong with the year) there was an change to the factory brake master cylinder and booster around '00 that upgraded them for disc brakes..... I am sure MrBlaine will know....
Since your Jeep originally came with drum brakes and both the front/back have upgraded disc brakes; the volume of brake fluid may not be enough. From your description... the master cylinder may be acting up...

 

[fizzlebottom]

Are there any original (or generally old) rubber hoses that are still part of your braking system? If so, they may be expanding (although not visibly) when the brake fluid gets hot.

Have you fully flushed the brake fluid? Bleeding is one thing, but fresh fluid all around will help rule it out altogether.

Is there any sediment in your master cylinder? If so, you may have pushed some into the lines and formed a clog that's reducing flow to one or more of the wheels. It could even be reducing flow to the rear proportioning valve.

 

[ApexOffRoad]

When we got back I did notice a small degree of "fraying" where the steel braided brake extensions on the front are rubbing on the steel braided line of the shock absorber. There's no visible fluid leak and I'm not sure if that condition was there before we hit the trail or if the trailer weight changed up the the articulation in the front to cause these to rub.

 

[Zorba]

Sounds like air - or maybe water - in the lines or master cylinder not reacting well to the heat. *shrug*

 

[ApexOffRoad]

Original brake system is 21 years old, anything is possible. With all the money I have spent perhaps I'm just better off starting over with the brake components. I do agree that the Master Cylinder may just be a bit overwhelmed why the addition of disc brakes all around and topped off by the weight of the trailer. That would make sense when you take into account that I had problems on the first day but on day four it was still recovering after brief moments of failure. seems like any of the other scenarios would have only gotten progressively worse? If you guys have not towed a trailer across a difficult trail with a TJ I'm here to tell you its a challenge. I'm an intermediate driver and without an expert spotter I never would have made it. My rig is very capable and lacking only in brute horsepower. As much as I love the TJ it's just not designed to handle the trailer. I also have a YJ that handles the trailer much better because of the leafs. It also has a spongy brake response when hooked up to the trailer but has never failed to stop like this scenario.

 

[Bill M.]

I would give Blaine at Black Magic Brakes a call. Blaine is always willing help even if the brakes are not his.

 

[Mike_H]

Maybe tagging him would help. I'm curious about this issue.

@mrblaine

 

[ApexOffRoad]

Thanks, appreciate all the feedback.

 

[Bill M.]

OP how do you like the Rubicon Express long arm with the Walkers shocks and Acos bump stops?

 

[ApexOffRoad]

Like I said, I'm an old guy and basically a newbie to the scene with marginal off road skills. I have a car dealership and access to some great mechanics who helped me build this TJ and also a YJ. Originally this started out as a father/son project but he went off to college before we could finish the project and I never got around to taking these rigs anywhere. Kids have moved back now and we are starting over with this Rubicon trip being our first major outing. All of that said, I was very happy with the components I have chosen. They might not all be the latest and greatest but this rig performed very well. Articulation was exceptional which led to better traction. I aired down to 12 lbs, Nothing broke under load but I'm not aggressive on the accelerator. I feel that without the trailer I would have breezed through the toughest obstacles. The trailer was responsible for more than a few white knuckle moments and if I'm being honest we did flip it over once (my fault, too much air in the trailer suspension made it bouncy) The only bypass we took on the whole trail was "Little Sluice" and most of those rigs were serious builds, buggy's or had 40" tires. All in all and outside of the braking issue I'm very happy with the performance of the combined parts. If any of you guys know Jeff at Arabias Overkill, he helped me with choices. Very good dude and knows his stuff.

 

[mrblaine]

Original brake system is 21 years old, anything is possible. With all the money I have spent perhaps I'm just better off starting over with the brake components. I do agree that the Master Cylinder may just be a bit overwhelmed why the addition of disc brakes all around and topped off by the weight of the trailer. That would make sense when you take into account that I had problems on the first day but on day four it was still recovering after brief moments of failure. seems like any of the other scenarios would have only gotten progressively worse? If you guys have not towed a trailer across a difficult trail with a TJ I'm here to tell you its a challenge. I'm an intermediate driver and without an expert spotter I never would have made it. My rig is very capable and lacking only in brute horsepower. As much as I love the TJ it's just not designed to handle the trailer. I also have a YJ that handles the trailer much better because of the leafs. It also has a spongy brake response when hooked up to the trailer but has never failed to stop like this scenario.

We need some more info on brake caliper size in the front. You don't have a vacuum problem. You do have 1 of 2 or maybe even a combination of both issues. 1 is very common at higher elevations with old fluid and that is vapor lock. There are 2 symptoms of vapor lock, one is working the rig hard, slow movement, lack of air flow under the hood and the brake components overheat and boil the fluid. The resulting steam pocket causes the pedal to go to the floor with zero resistance. The second symptom is the brakes come right back to full functionality in a few minutes as soon as it cools under the hood just a bit.

The other possibility is caliper piston knockback. That is caused by a loose unit bearing, loose rim, bent rear axle or similar and that is pushing the caliper piston in so that you have to pump it back out. That can be diagnosed by just pumping the pedal several times and getting brake function back until it happens again. Idling the rear axle on jack stands and watching the caliper move side to side will also reveal it.

You'll have to see what fits best.

 

[ApexOffRoad]

Thanks mrblaine, That's solid advice and very much appreciated. Both of my Jeeps have been sitting for quite some time with only occasional use to keep the batteries charged. I will report back after a fluid change and caliper inspection.

To each of you that replied thanks for the input.

Gary