Brake Expert Needed

[pcoplin]

I would think that motor should have 19-20 in vacuum at sea level. Have you checked for any intake manifold leaks? Doubt it's high mileage, but high miles can also cause low vacuum.

edit: You using the 3/8 port on the rear of the manifold?

 

[Ericict]

I have a crate 6.2 LS3 430. Stock cam.

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My wife said no, but I can do it when she dies! It’s times like this that make life difficult.

 

[Ron Hall]

Going back to your first description of the problem, it sure points suspicion of a bad master cylinder. When the rear brakes released did the pedal drop any at all?

 

[Rooster Feather]

I would think that motor should have 19-20 in vacuum at sea level. Have you checked for any intake manifold leaks? Doubt it's high mileage, but high miles can also cause low vacuum.

edit: You using the 3/8 port on the rear of the manifold?

Motor was brand new. Only about 2,500 miles on it now.

 

[Rooster Feather]

Going back to your first description of the problem, it sure points suspicion of a bad master cylinder. When the rear brakes released did the pedal drop any at all?

During this testing I replaced the Master Cylinder. No change.

 

[Ron Hall]

During this testing I replaced the Master

 

[Ron Hall]

Good Luck finding the cause. .

 

[mrblaine]

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.

While the pressure test would be beneficial, we already know that the pressure is high enough to stop the rear tires and without the pedal moving, they start turning again. That means the pressure is being lost.

A caliper piston problem wouldn't produce that result unless there was a pinhole leak which let the pressure bleed off after the initial stop. He has found no leaks of any kind. A sticky or stuck piston would either hold and keep holding or not hold at all being stuck in the bore. I can't see any other piston issues that would cause it to grab and then release.

I don't see how air in the rear circuit will cause it either. All that does is equate to overall lower pressure and a reduction in clamping force.

So far, the only correlation in pressure loss is when the motor has over 17 inHg, the system has enough pressure to stop the rear and hold. When it is under that, it doesn't.

 

[Fouledplugs]

I may be way out of my league here, I have a lot to learn on TJ brakes.

Wouldnt a “bigger”/internally upgraded PS pump and hydroboost eliminate the need for vacuum thus eliminating the reliance on engine vacuum?

 

[qslim]

So far, the only correlation in pressure loss is when the motor has over 17 inHg, the system has enough pressure to stop the rear and hold. When it is under that, it doesn't.

Look to the brake booster then? What kind of vacuum is the factory brake booster set up for? I haven't had occasion to check vacuum on a 4.0, is it appreciably more than what the OP is getting from his LS?

Has anyone considered a leaking diaphragm in the booster?

 

[mrblaine]

Look to the brake booster then? What kind of vacuum is the factory brake booster set up for? I haven't had occasion to check vacuum on a 4.0, is it appreciably more than what the OP is getting from his LS?

Has anyone considered a leaking diaphragm in the booster?

The 4.0 is rarely below 20 inHg. I did consider a leaking diaphragm or both. I suggested a swap from another vehicle with a known good booster to rule it out even though I can't come up with what symptom a leaking booster diaphragm would produce. I am not as well versed with regard to the intricacies of how a booster functions versus the rest of the system which I know very well. A loss in vacuum would cause a harder pedal which is not present. If there is a defective diaphragm, it should present as an across the board loss in system force at the caliper, not as one that is high and then drops.

The correlation between the engine vacuum reading and the release of the rear caliper that lets the wheels turn after stopping points to it being vacuum related, not booster related. That and the fact that the fronts work well or are reported to also points away from the booster. I would however, still like to see a known good one swapped in just to rule it out completely. Not new, known good.

 

[mrblaine]

I may be way out of my league here, I have a lot to learn on TJ brakes.

Wouldnt a “bigger”/internally upgraded PS pump and hydroboost eliminate the need for vacuum thus eliminating the reliance on engine vacuum?

If I had my way, all brakes in the TJ would be dual piston front calipers driven by hydroboost.

 

[qslim]

The 4.0 is rarely below 20 inHg. I did consider a leaking diaphragm or both. I suggested a swap from another vehicle with a known good booster to rule it out even though I can't come up with what symptom a leaking booster diaphragm would produce. I am not as well versed with regard to the intricacies of how a booster functions versus the rest of the system which I know very well. A loss in vacuum would cause a harder pedal which is not present. If there is a defective diaphragm, it should present as an across the board loss in system force at the caliper, not as one that is high and then drops.

The correlation between the engine vacuum reading and the release of the rear caliper that lets the wheels turn after stopping points to it being vacuum related, not booster related. That and the fact that the fronts work well or are reported to also points away from the booster. I would however, still like to see a known good one swapped in just to rule it out completely. Not new, known good.

That was my line of thought as well.. I know a 'bad' or leaking booster would present an issue across the board as well as a stiff pedal, but a customer's perception of what a stiff pedal is w/ a weakened booster could be totally different than someone like you who lives and breathes (hopefully not literally) brakes for a living.

So, going back to the swap, is it not common or good practice to install a bigger brake booster as well when installing a low-vacuum (comparatively) engine like an LS? Say you had a delta in engine vacuum @ idle of around 4 psi between the stock jeep engine and the LS, that could represent a measurable loss in assist from the factory booster.

 

[mrblaine]

That was my line of thought as well.. I know a 'bad' or leaking booster would present an issue across the board as well as a stiff pedal, but a customer's perception of what a stiff pedal is w/ a weakened booster could be totally different than someone like you who lives and breathes (hopefully not literally) brakes for a living.

So, going back to the swap, is it not common or good practice to install a bigger brake booster as well when installing a low-vacuum (comparatively) engine like an LS? Say you had a delta in engine vacuum @ idle of around 4 psi between the stock jeep engine and the LS, that could represent a measurable loss in assist from the factory booster.

If you haven't tried to install a bigger booster, the challenges are not fully understood. The only candidates are the auto versions since the manual clutch master sits too close to increase the diameter of the existing booster in any meaningful manner. The easier solution would be an aux vacuum supply via pump or similar.

 

[Dan Carey]

I would check where the vacuum lines are connected on the replacement engine in its intended vehicle. There are several places a line could be attached and get different readings.

 

[Rooster Feather]

To see if the low vacuum problem was why my rear brakes didn't work, I hooked my Dodge pickup to my Jeep brake booster. The vacuum output of my pickup was 18-20 inHg. I tested the Jeep on jacks again and no change to the effectiveness of my braking. So it is evidently not a low vacuum issue.

 

[mrblaine]

To see if the low vacuum problem was why my rear brakes didn't work, I hooked my Dodge pickup to my Jeep brake booster. The vacuum output of my pickup was 18-20 inHg. I tested the Jeep on jacks again and no change to the effectiveness of my braking. So it is evidently not a low vacuum issue.

I'm reminded of one more thing to try. Pull the plunger and spring out of the front part of the combo block again. Find or make a washer that is about 1/16" or so thick that fits onto the plunger to add some tension to the spring. Be careful not to let the OD of the washer exceed the plunger piston diameter where the spring sits. I've used banjo bolt crush washers before and just sanded down the OD to get it the right size.

 

[TJ Ben]

I have a crate 6.2 LS3 430. Stock cam.

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Wow that’s a nice swap! Any info on the swap?

 

[Rooster Feather]

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!

 

[Fouledplugs]

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!

Thank you for the follow up. Makes future searches more beneficial.