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Old 07-01-2019, 08:45 PM   #1 (permalink)
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Brake Expert Needed!!

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.
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Old 07-01-2019, 10:04 PM   #2 (permalink)
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Drum brakes suck going backwards. Weight transfer, shoe pivot, and some other magic are to blame. Disc brakes don't seem to care. When you're going backwards down a hill, the rear (which has less braking ability) now has more weight pushing down on them. The front brakes now have less weight holding them down will lock up. Try backing up on flat ground and stabbing the brakes. Same thing will happen.

Vacuum...provides no holding power. It only assists you in depressing the pedal, thus pushing more fluid.

Frankly, if you have an LS wrangler with bigger tires, its time for bigger brakes anyway. Good Luck
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Old 07-01-2019, 10:23 PM   #3 (permalink)
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Quote:
Originally Posted by Rooster Feather View Post
My equipment is a 2005 TJ Rubicon, 35" tires, 4 wheel stock disc brakes, LS motor.
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Originally Posted by nkldrgr View Post
Drum brakes suck going backwards.


How old are your brake hoses? Braided or rubber?

Any modifications to your motor? Cam “upgrades”?
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& Blaine


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Old 07-02-2019, 04:55 AM   #4 (permalink)
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I feel bad for Blaine...…..
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Old 07-02-2019, 07:41 AM   #5 (permalink)
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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.
Jesus.
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Old 07-02-2019, 10:06 AM   #6 (permalink)
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I feel bad for Blaine...…..
Yep, but great advertising for his customer service if this is the follow through he gives. Not clear that the OP actually bought anything from him, and still all of this effort.
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Old 07-02-2019, 10:36 AM   #7 (permalink)
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Another option.....mod the Tcase for a 2wd low option. That way you can toss it into 2Lo and use gearing to help slow the vehicle......4wd won't be needed in that scenario for stopping power.....of which the braking system was setup for a 4.0. Now that an LS has been planted the braking system doesn't know how to respond to pedal effort.

You bought it with the LS already swapped in didn't you...…..
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Old 07-02-2019, 11:01 AM   #8 (permalink)
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The rear caliper pistons are not stuck in their bores?

ABS equipped?

If the engine has low vacuum, you can add in an electric vacuum pump (A lot of late model vehicles including JK's run them) or switch to Hydro-boost.

Last edited by Marty, SoCal; 07-02-2019 at 11:05 AM.
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Old 07-02-2019, 05:50 PM   #9 (permalink)
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Rear calipers are reversed L/R making your bleeder facing the wrong direction (down)???????
Shot in the dark!!!!


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Old 07-02-2019, 05:59 PM   #10 (permalink)
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WTF is inverted vacuum?
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Old 07-02-2019, 09:34 PM   #11 (permalink)
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WTF is inverted vacuum?
That's boost.
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Old 07-03-2019, 12:49 PM   #12 (permalink)
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I feel bad for Blaine...…..
You think you feel bad....you should be the guy calling him. He did give me his personal cell though. He has been amazingly helpful.

Quote:
Originally Posted by Beat95YJ View Post


How old are your brake hoses? Braided or rubber?

Any modifications to your motor? Cam “upgrades”?
Steel brake lines. Crate 6.2 LS3 430, stock cam

Quote:
Originally Posted by Kunker View Post
Yep, but great advertising for his customer service if this is the follow through he gives. Not clear that the OP actually bought anything from him, and still all of this effort.
He knows that I am going to purchase the Big Brake system as soon as this is figured out.

Quote:
Originally Posted by cj8scrambld View Post
Another option.....mod the Tcase for a 2wd low option. That way you can toss it into 2Lo and use gearing to help slow the vehicle......4wd won't be needed in that scenario for stopping power.....of which the braking system was setup for a 4.0. Now that an LS has been planted the braking system doesn't know how to respond to pedal effort.

You bought it with the LS already swapped in didn't you...…..
I had RPM Extreme to the LS conversion after I owned the Jeep.

Quote:
Originally Posted by Marty, SoCal View Post
The rear caliper pistons are not stuck in their bores?

ABS equipped?

If the engine has low vacuum, you can add in an electric vacuum pump (A lot of late model vehicles including JK's run them) or switch to Hydro-boost.
Calipers are not stuck, no ABS.

Quote:
Originally Posted by gregj50 View Post
Rear calipers are reversed L/R making your bleeder facing the wrong direction (down)???????
Shot in the dark!!!!
Nope!

After researching this I have found that it is not uncommon for the LS motors to have low vacuum at idle. This would explain why, while sitting on jack stands, the rear tires would turn when I was on the brake and put it in drive. It also explains why, when sitting on the jack stands and I accelerated to 20-30 mph the rear tires would stop, then start to turn again when the motor dropped to idle. It takes 17 inches of vacuum to make the booster function. Unless there is a trick to get the vacuum to increase at low rpm I will have to get an auxiliary vacuum pump I guess.

Thank you for all the response!
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Old 07-03-2019, 01:11 PM   #13 (permalink)
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Weird.....any power brake equipped vehicle I've owned would stop without the motor running.....ie: ZERO vacuum....it just takes way more pedal effort, but it would stop.
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Old 07-04-2019, 04:31 AM   #14 (permalink)
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With that motor you might want to put some real axles and brakes under your rig before you waste any more time on these.
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Old 07-07-2019, 11:49 PM   #15 (permalink)
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Master cylinder?
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Old 07-08-2019, 08:51 AM   #16 (permalink)
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Master cylinder?
Took off the one that was on it. Bench bled it, reinstalled it, and re-bled the lines. No change. Bench bled a new one, installed it, and re-bled the lines. Still no change!
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Old 07-08-2019, 08:54 AM   #17 (permalink)
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With that motor you might want to put some real axles and brakes under your rig before you waste any more time on these.
??? It's got Dana 44's with Revolution Gear Chromemoly axles. I'm going to put a Vanco big brake system on it, but we have to figure out the rear brakes first.
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Old 07-09-2019, 10:19 AM   #18 (permalink)
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??? It's got Dana 44's with Revolution Gear Chromemoly axles. I'm going to put a Vanco big brake system on it, but we have to figure out the rear brakes first.
Out of curiosity, why? At what point do you really care about rear brakes this much? Does it work while driving okay? It sounds like it's only a "problem" on jack stands. Have you put a pressure gauge on the lines at the rear? Is it bleeding off pressure?

I'd say throw the new brake kit on and go wheeling, but that's me.
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Old 07-10-2019, 08:57 AM   #19 (permalink)
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It sure sound like you have an internal leak in the system from the way you describe it losing pressure. First thought would be master cylinder which you said you have replaced. Not sure how ABS system works, but I suspect that may be a culprit as well. Get a friend to press the brakes for you and follow the rear lines from the master to the calipers and see if any soft lines are bulging or something looks suspect. That or try to read up on how the ABS works. Thats all I can think of. Good luck.
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Old 07-10-2019, 10:56 AM   #20 (permalink)
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Where are you measuring vacuum from? Does it change with engine temp at all? Do you have the stock pvc system on it? Also you could just switch to a hydroboost system and be done with it...
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Old 07-10-2019, 01:03 PM   #21 (permalink)
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Lots questions are asked and answers are sought on forums, but many times when the matter has been resolved the solution is not posted. This is a summary of the procedure and the resulting solution to my problem.



Issue: Weak Rear brakes. While on jack stands, in gear at an idle, the rear brakes could not keep the rear tires from turning. At a speed of 20-30 mph (on the jack stands) the brakes would stop the tires but without letting off the pedal, the rear tires would begin to spin again.



1. Checked brake lines and Calipers for any crimps or leaks.

2. Bled brakes then tested. No change.

3. Re-bled brakes then tested. No change, called Blaine.

4. Turned key on and bled brakes clearing the Brake-Light warning after each pedal push removing air, then tested. No change, called Blaine.

5. Checked brake fluid pressure at Master Cylinder's two outputs and the three combination valve outputs then tested. No change, called Blaine.

6. Removed Master Cylinder, bench bled, replaced then tested. No change, called Blaine

7. Purchased new Dorman Master Cylinder, bench bled, installed, and tested. No change, called Blaine

8. Removed rubber cap from the combination valve plunger, re-bled then tested. (Rear brakes worked better, but the pedal was spongy.

9. Replaced rubber cap to combination valve plunger and re-bled.

9. Checked vacuum at the brake booster (14-28 in HG). Hooked up vacuum line from my 16' Ram 1500 (20 inHg) then tested. No change, called Blaine and Jon Schaefer from RPM Extreme.

10. Replaced brake booster with a brake booster from buddies TJ then tested. No change, called Blaine.

11. Used a 3/32" cooper washer in combination valve to compress spring, re-bled then tested. Success! Called and thanked Blaine.

12. Ordered VANCO Big Brake Kit, rear Centric rotors, rear Black Magic Brake pads, and an orange Master Cylinder Cap from Black Magic Brakes!!



Needless to say, Blaine was a HUGE help. I placed an order with him today, however he informed me that he was happy to help and would have helped if I would have never placed or intended to place an order with him.

Thank you to everyone that gave input. Special thank you to Blaine Johnson of Black Magic Brakes.

Last edited by Rooster Feather; 07-10-2019 at 01:07 PM.
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Old 07-16-2019, 09:28 PM   #22 (permalink)
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My experience with LS motors and any car sized brakes( cj,yj,tj and explorer 8.8’s is that the low 1St gear in a 4L60e and torque of a 5.3 or above easily goes thru the rear brakes even at idle.
So dont waste time with master cylinders or vacuum dohickys or other stuff, put in a psc power steering pump and a hydroboost II(2) and braided stainless hoses and enjoy brakes that hold going down hill and steering that doesnt lock up and u r prepared for the future.

And Do not put an adjustable proportional valve on the front lines,brakes 101, scares me someone would suggest that in the first place.

Let the flames begin
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