Brake Expert Needed

[Rooster Feather]

My equipment is a 2005 TJ Rubicon, 35" tires, 4 wheel stock disc brakes, LS motor

I was backing down my incline gravel driveway and my front tires locked up and the rear tires continued to roll. I did not think this was normal so I posted for help on the forum. I also contacted Blaine Johnson, Mr. Black Magic himself.

Blaine told me to bleed the brakes and put the rear axle on jack stands, accelerate to 30 mph and apply the brakes. If it stops the tires it is working.
As soon as I pressed the brakes and put it in drive the rear tires started turning. I couldn't even stop them without even applying the throttle. I went ahead and accelerated to 30 mph and hit the brakes and the tires stopped momentarily then started to roll again while I had the brakes depressed.

I called Blaine back and reported it. He said that I had air in my lines. I re-bled the brakes and repeated the test. No change. I called Blaine. He said that I need to repeat the test and without letting off the brake I needed to crack open the lines on the brake master cylinder and combination valve one by one and see if I fluid came out under pressure. We were trying to find where the pressure was dropping. He believed it maybe a bad master cylinder. I did, tested with the exact same result.

I called Blaine again. He said to remove the master cylinder, bench bleed, re-bleed the brakes and retest. I did. No change. Tires stop at 30 mph momentarily then begin rolling again. I then purchased a Dorman master cylinder (Blaine's choice), bench bled it, installed it, and re bled the brakes. Tested it. No change.

I called Blaine again. He said to remove the plunger our of the combination block and remove the rubber/plastic ring. I did, re bled the brakes, re tested and this time the tires stopped, then rolled over a couple of times then stopped again. But the brake pedal was extremely spongy, which Blaine warned me it might be.

He thought that maybe the combination block was bad. He told me to get a new combination block and retest. I asked him if I should get an adjustable proportional valve. He said absolutely not! That while it does allow you to put more towards the rear, it restricts fluid to the front brakes which is never a good thing.

I hung up, thought about everything he said and called him back. I told him that I had an LS motor and not the stock 4.0. I was wondering if the torque of the motor was pushing through the brakes. Blaine believed that maybe the vacuum going to the brake booster was too low. So I reinstalled the rubber/plastic ring back on the plunger in the combination block and retested and tested the vacuum pressure in the line going to the brake booster. What I found was that when I would increase the rpm the vacuum would go up to 20-22 inches. When I let off the accelerator and hit the brakes the vacuum would drop down to 13-15 then increase up to 17 inches while at an idle. We determined that when the vacuum was below 17 inches of mercury the brakes would not be enough to hold the tires at a stop. Blaine said that I had somewhat of an anomaly because in all of his experiences the vacuum decreased with higher rpm and increased with lower rpm and mine was just the opposite. He recommended that I research this vacuum issue.

Blaine told me that we are only trying to make the brakes stop the tires and hold. We are not trying to apply more pressure to the rear. He said that just stopping the rear tires is all that is expected from a Jeep. I don't understand that. Why would Jeeps be made this way. How can I drive it in 2wd if my rear brakes only have enough power to stop my tires when they are on jack stands? It would not be reasonable to put my Jeep in 4wd every time I started to back down my driveway.

Has anyone had this issue with their brakes and/or a low/inverted vacuum pressure?

BTW my plan is to install the VANCO Big Brake Kit, but Blaine said that we have to fix the rear brakes first.

 

[K0LCB]

Have you considered adding a proportional valve?

 

[mrblaine]

Have you considered adding a proportional valve?

Yes, he already said that and we discussed it. Aftermarket adjustable prop valves don't have a knee point. They just reduce the pressure by a percentage across the range. The stock one lets the pressure rise as a percentage under hard braking and the it breaks over and it rises at a slower rate.

We ran the gamut of tests and possible causes for why the brakes would be releasing after the initial stop. What was happening didn't make any sense on the hydraulic side. The pulling of the seal on the prop section was to point us to anything that would make a change in what was happening to narrow it down.

 

[qslim]

It doesn't sound like you have a vacuum issue... I don't have any experience w/ the LS engine specifically but a vacuum reading of 17 in hg isn't terribly low...Rule of thumb is about 17-21 in hg at idle, subtracting around 1" for every 1000 feet above sea level. When you open the throttle you should see a momentary drop in vacuum as the engine accelerates followed by a increased steady reading, which is what you described.

I would suggest changing the combination block, it sounds like something is restricting hydraulic pressure to the rear calipers.

 

[andy29847]

There is lots of info on the internet similar to what you describe. https://www.jkowners.com/forum/modified-jk-tech-dept/323921-gm-ls-engine-brake-vaccum.html

 

[mrblaine]

My equipment is a 2005 TJ Rubicon, 35" tires, 4 wheel stock disc brakes, LS motor

I was backing down my incline gravel driveway and my front tires locked up and the rear tires continued to roll. I did not think this was normal so I posted for help on the forum. I also contacted Blaine Johnson, Mr. Black Magic himself.

Blaine told me to bleed the brakes and put the rear axle on jack stands, accelerate to 30 mph and apply the brakes. If it stops the tires it is working.
As soon as I pressed the brakes and put it in drive the rear tires started turning. I couldn't even stop them without even applying the throttle. I went ahead and accelerated to 30 mph and hit the brakes and the tires stopped momentarily then started to roll again while I had the brakes depressed.

I called Blaine back and reported it. He said that I had air in my lines. I re-bled the brakes and repeated the test. No change. I called Blaine. He said that I need to repeat the test and without letting off the brake I needed to crack open the lines on the brake master cylinder and combination valve one by one and see if I fluid came out under pressure. We were trying to find where the pressure was dropping. He believed it maybe a bad master cylinder. I did, tested with the exact same result.

To clarify, we discussed that we did not actually believe the master was bad since there is no way for the hydraulic pressure to bleed off internally due to the way a master is designed and I did not believe that a swap would make any difference in what was happening and it didn't.

I called Blaine again. He said to remove the master cylinder, bench bleed, re-bleed the brakes and retest. I did. No change. Tires stop at 30 mph momentarily then begin rolling again. I then purchased a Dorman master cylinder (Blaine's choice), bench bled it, installed it, and re bled the brakes. Tested it. No change.

As predicted.

I called Blaine again. He said to remove the plunger our of the combination block and remove the rubber/plastic ring. I did, re bled the brakes, re tested and this time the tires stopped, then rolled over a couple of times then stopped again. But the brake pedal was extremely spongy, which Blaine warned me it might be.

He thought that maybe the combination block was bad. He told me to get a new combination block and retest. I asked him if I should get an adjustable proportional valve. He said absolutely not! That while it does allow you to put more towards the rear, it restricts fluid to the front brakes which is never a good thing.

Sort-of. I explained that we don't understand how it can release pressure since at no point in time is the rear pressure higher than the front in the combo block. Since there was no evidence of external leaks, that would mean the rear pressure would have to be bleeding off into somewhere and it really can't do that. I don't believe the combo block is bad and as I said, we are or were looking for anything to make a change since all the other tests were null for anything that would point to the pressure dropping and releasing the wheels.

I hung up, thought about everything he said and called him back. I told him that I had an LS motor and not the stock 4.0. I was wondering if the torque of the motor was pushing through the brakes. Blaine believed that maybe the vacuum going to the brake booster was too low. So I reinstalled the rubber/plastic ring back on the plunger in the combination block and retested and tested the vacuum pressure in the line going to the brake booster. What I found was that when I would increase the rpm the vacuum would go up to 20-22 inches. When I let off the accelerator and hit the brakes the vacuum would drop down to 13-15 then increase up to 17 inches while at an idle. We determined that when the vacuum was below 17 inches of mercury the brakes would not be enough to hold the tires at a stop. Blaine said that I had somewhat of an anomaly because in all of his experiences the vacuum decreased with higher rpm and increased with lower rpm and mine was just the opposite. He recommended that I research this vacuum issue.

Blaine told me that we are only trying to make the brakes stop the tires and hold. We are not trying to apply more pressure to the rear. He said that just stopping the rear tires is all that is expected from a Jeep. I don't understand that. Why would Jeeps be made this way. How can I drive it in 2wd if my rear brakes only have enough power to stop my tires when they are on jack stands? It would not be reasonable to put my Jeep in 4wd every time I started to back down my driveway.

Again, sort-of. To move forward with the installation of the brake kit, we are looking to solve the first problem which is the tires releasing after the initial stop. We need to get the system to stop allowing the rear tires to release before you consider the BBK because they will still be doing that after the install if we don't find out why that is happening. So, we need to get the backs to stop and hold first before you do anything else.

To clarify. Nearly all car braking system achieve a front to rear bias with lower "power" rear brakes in two ways. They use components that are roughly half the size of the fronts and they run a proportioning valve in the system to limit rear pressure to prevent lock up.

That can be found by looking up the component sizes on places like Rock Auto. For example, a TJ front is a 66mm caliper with a large brake pad. The rear is a 48mm caliper with a pad that is about 1/3 the size. A 66mm is 5.22" of piston area and a 48mm is 2.8" of piston area. The front rotor is vented, the rear rotor is solid. Vented are used where they do a lot of work, solid are used where they don't do much work. [/quote]

Has anyone had this issue with their brakes and/or a low/inverted vacuum pressure?

BTW my plan is to install the VANCO Big Brake Kit, but Blaine said that we have to fix the rear brakes first.
[/QUOTE]

To clarify, the vacuum decreases under acceleration. It should be above 17 at idle. It will move up as the RPM increases but drop under accel. If you install a temporary vacuum gauge that should show that.

It should be noted that the we in this is me discussing this with the only other person I know that understands how brakes work as well as I do. After many conversations we arrived at the conclusion that there is something glaringly obvious that the owner sees and knows that hasn't been related that will point us towards what is going on. After 20 phone calls and lots of conversation the "AHA!" moment was the LS swap which are notorious for vacuum issues.

 

[Ericict]

Blain is correct about low vacuum. Should be >15hg. Where are you taking the reading? Is it isolated at the engine?

 

[andy29847]

It should be noted that the we in this is me discussing this with the only other person I know that understands how brakes work as well as I do. After many conversations we arrived at the conclusion that there is something glaringly obvious that the owner sees and knows that hasn't been related that will point us towards what is going on. After 20 phone calls and lots of conversation the "AHA!" moment was the LS swap which are notorious for vacuum issues.

Mr. Blaine, for someone so helpful you sure do catch a lot of shit. Don't worry, I love you.

 

[mrblaine]

Mr. Blaine, for someone so helpful you sure do catch a lot of shit. Don't worry, I love you.

I'm slightly disappointed in that he makes it sound like all I did was tell him to start swapping parts out in a shotgun approach. Actually I talked him through how each part functions and why a test of each part might point us to discover why the rear brakes are releasing. We would not have known that had we not started with the basics. Make sure the system is bled properly with the key on and watch the brake light on the dash. Once we are sure the system is free of air, lets walk through a diagnostic process and check each component along the way to see if we can find something that will point to why the rears are releasing. The master was done as an answer to "do you know the provenance of that part?" and the resultant no answer. We didn't know if it had been swapped which means that it needed to be bench bled for two reasons. First is to make sure there is no air in the master and the second is to check for fluid leaking into the booster which will kill the booster.

The one thing that stuck was the fact that the pedal did not drop or move but the rears would stop turning initially and then release and start turning. This was after it was verified that there were no visible leaks anywhere in the system. That means that something was allowing the pressure into the calipers to drop and without a leak somewhere, that is almost impossible hydraulically.

He and I share a common problem having a steep gravel driveway. If I slow down and try to ease up mine, I spin the rear tires and kick gravel out into the street which I have to go sweep up and put back in the driveway. I solve that by either zipping around the turn or I reach down and pop it into 4 hi, no tire spin, no gravel in the street. The same is true for backing down it and needing to stop before I get into the street. If I pop it into 4 hi, the front brakes help the rear brakes work to stop without skidding.

If you've ever driven an older pick-up without a load sensing brake bias adjuster or rear ABS, it is hard to understand why the rear TJ brakes are dialed out so much. On those trucks running empty, it is impossible to stop them from locking up the rear brakes. Not so bad in a longer wheelbase, very bad on the short wheelbase Jeeps though.

 

[qslim]

If you've ever driven and older pick-up without a load sensing brake bias adjuster or rear ABS, it is hard to understand why the rear TJ brakes are dialed out so much. On those trucks running empty, it is impossible to stop them from locking up the rear brakes. Not so bad in a longer wheelbase, very bad on the short wheelbase Jeeps though

Interesting. So given than do you think the OP's original observation and reason for concern may not have been too far out of the ordinary?

 

[mrblaine]

Interesting. So given than do you think the OP's original observation and reason for concern may not have been too far out of the ordinary?

Two things pointed to an anomaly. The first was a lack of highly visible wear pattern on the rear rotors and the test showing that they initially stopped and then started turning when run on jack stands. The wear pattern was the first indication of something abnormal which was looked at due to the fronts locking up. So not that far out of the ordinary. It is also very hard to tell what is going on when you have to depend on the observations of others.

Had the LS swap been at the forefront of the diagnostic process, the entire thing would have been different. I get lots of calls from folks who ONLY did a swap and with no other changes, there is now a markedly different brake performance and response. The cobbled together solutions for getting vacuum from the motor to the things on the TJ that need vacuum are very often problematic. Not saying this one is cobbled, but many others have been so I know to start there so it can be ruled out.

 

[Dan Carey]

First, I don't know whats wrong with your brakes. I do know that when I was racing big block chevys with auto transmissions we didn't have brakes worth a crap at an idle. The engine needed to be at 1500 and above to have any vacuum at all. Big cams have too much overlap to create vacuum. Good luck.

 

[astjp2]

What about adding a vacuum pump or accumulator? I used to have a set of high pressure gauges, you remove the bleeder, put them into the bleed port, hit the brakes and watch the gauges. Good troubleshooting tool but not used often enough to keep.

 

[Robin Down]

Maybe this has already been checked, but could the rear calipers be causing the issue by not applying the pressure evenly, if at all?

 

[astjp2]

they do stop for a short time so what does that tell you? The OP should have gone with a 4.6 stroker? Sorry, I could not resist the sarcasm. I am believing it is a problem with the rear caliper piston, or there is air in the system. a real pressure test with a gauge would help troubleshoot the problem.

 

[Rooster Feather]

It should be noted that the we in this is me discussing this with the only other person I know that understands how brakes work as well as I do. After many conversations we arrived at the conclusion that there is something glaringly obvious that the owner sees and knows that hasn't been related that will point us towards what is going on. After 20 phone calls and lots of conversation the "AHA!" moment was the LS swap which are notorious for vacuum issues.

Absolutely correct! It was me, someone with extremely limited brake knowledge, trying to describe my braking problem via telephone to "the" TJ brake guru. Blaine has been amazingly helpful and extremely gracious to help me find my problem!

It should also be noted that the original post was paraphrased from our complete conversation. There is not an "official" transcript of those actual phone call conversations. If in fact, the actual conversations were typed....I would have sounded much more ignorant than I really am.

In the last conversation we had, Blaine suggested I research the internet and try to find out if any other TJ Jeep LS conversion owners have had any vacuum problems. That was the reason for this post. If I did not outline what was already tested, we would have re-lived it. Just trying to avoid another "AHA" moment.

I hope that this post did not sound like I doubted or was unappreciative of Blaine. I am 100% team Blaine and he absolutely knows brakes. If he tells me to throw gas and a match on it I would because I know in some way that would help!! Blaine, I can not thank you enough. I'll be calling ......(bet you can't wait!)

 

[mrblaine]

Absolutely correct! I should have not used "we" because that was incorrect. It was me, someone with extremely limited brake knowledge, trying to describe my braking problem via telephone to "the" TJ brake guru. Blaine has been amazingly helpful and extremely gracious to help me find my problem!

The last conversation we had Blaine suggested I research the internet and try to find out if any other TJ Jeep LS conversion owners have had any vacuum problems. That was the reason for this post. If I did not outline what was already tested, we would have re-lived it. Just trying to avoid another "AHA" moment.

I hope that this post did not sound like I doubted or was unappreciative of Blaine. I am 100% team Blaine and he absolutely knows brakes. If he tells me to throw gas and a match on it I would because I know in some way that would help!! Blaine, I can not thank you enough. I'll be calling ......(bet you can't wait!)

If I didn't want to help, I wouldn't have given you my pocket phone number, call when you need to.

 

[Chris]

@pcoplin, you've got an LS in your TJ.

 

[pcoplin]

What LS engine do you have? Is it cammed?

 

[Rooster Feather]

What LS engine do you have? Is it cammed?

I have a crate 6.2 LS3 430. Stock cam.