poor braking at low rpms --

[jmg222]

turning my attention back to my 88 xj... I'm running 35's, front axle is a d30 w/ 27 spline tru track (was done so long ago I forget what else we did -- definitely re geared to 4.11). Running a vanco brake upgrade for 15" wheels, and a late model master/booster.

At low rpm, brakes are really sub par. tough to maintain going downhill, going up hill if I need to hold in place, I have to mash with both feet to get it to grip.

Any thoughts? I was looking potentially at putting in a d44, but they apparently use the same brakes. When moving above idle, brakes are adequate.

 

[IslanderOffRoad]

If it’s rpm related I’d suspect a vacuum leak at the booster

 

[jmg222]

If it’s rpm related I’d suspect a vacuum leak at the booster

thats what I thought, did a vacuum check, everything is fine.
I'm not sure it's rpm related -- maybe shouldn't have focused on that. Braking is less than adequate at low rpm, and feels almost ok when cruising, but I haven't tried stomping on it when cruising.

in the sticky on this forum on axel swaps, it says that braking becomes horrible with a built d30, but doesn't explain why.. is there something inherent in the setup that makes braking terrible?

This is where I know I’m going to catch some flack. I am not a very big believer in the Dana 30 for rock use. Jes and I have a pile of broken shafts and a broken carrier to prove it. So, I cannot recommend running a tire larger than 33 inches with a locker in the rocks. Larger than this, and several things become very apparent. Your braking ability becomes horrible,the unit bearings really begin to suffer, the ring and pinion are screaming for mercy and you may start having ball joint troubles.

 

[Black1990jeep]

New rotors and pads can help if yours are subpar. 35 inch tires dont help the braking, they are harder to stop. 4.11 gears is not ideal for 35 tires. I got 4.11 on 33 tires and it is not the best, for off road I even have a 1st gear lock switch added to prevent shifting to 2nd, that helps with engine braking.

I also have rear disc brakes added on a Ford rear end.

part of your issue maybe the gearing mismatch, part of your issue maybe the huge tires, which are harder to stop. have stock rear drums is not helpful on such a modified truck.
you may wish to get the rotors turned if they are grooved or glazed, and put some high performance pads on them. a rear disc conversion maybe good todo also, and correct gearing

but make sure your system is fully working too.

 

[Black1990jeep]

PS you did a vacuum check, what did you do? pull a vacuum on the booster and thus check for a booster leak? did you run a manifold vacuum test?

you might have poor engine vacuum, I had that do to a leaky egr, and this harmed braking and engine power, and made the brakes horrible as I had reduced vacuum booster.

now a small leak in the booster can have a bigg effect on brakes but may not show that much up on a manifold vacuum test.

I recommend you run both a manifold vacuum test and a leak test on the booster, if you havent done both already.

 

[jmg222]

New rotors and pads can help if yours are subpar. 35 inch tires dont help the braking, they are harder to stop. 4.11 gears is not ideal for 35 tires. I got 4.11 on 33 tires and it is not the best, for off road I even have a 1st gear lock switch added to prevent shifting to 2nd, that helps with engine braking.

I also have rear disc brakes added on a Ford rear end.

part of your issue maybe the gearing mismatch, part of your issue maybe the huge tires, which are harder to stop. have stock rear drums is not helpful on such a modified truck.
you may wish to get the rotors turned if they are grooved or glazed, and put some high performance pads on them. a rear disc conversion maybe good todo also, and correct gearing

but make sure your system is fully working too.

rotors and pads are brand new, vanco upgrade rotors/black magic pads
Rear is a ford 8.8, drum brakes, not stock (drums stop just as well as the discs, especially at low rpm).

gearing is actually fine -- both fronts and rears are 4.11, why do you say it's not correct?

 

[jmg222]

PS you did a vacuum check, what did you do? pull a vacuum on the booster and thus check for a booster leak? did you run a manifold vacuum test?

you might have poor engine vacuum, I had that do to a leaky egr, and this harmed braking and engine power, and made the brakes horrible as I had reduced vacuum booster.

now a small leak in the booster can have a bigg effect on brakes but may not show that much up on a manifold vacuum test.

I recommend you run both a manifold vacuum test and a leak test on the booster, if you havent done both already.

it's been awhile, I'll re do it. Again, I shouldn't have focused on the low rpm aspect of this -- the brakes are OK while cruising, not great, I just notice it mostly at lower rpms on the trail.

 

[Evan03]

Youl likely not see any improvement with swapping to disc rear.

On mine ive swapped to disc rear and saw little to no improvement over drums. Id say I have mediocre brakes at best.

One if my first thoughts is that there's air in the system. Seems there always is no matter how badass a brake bleeder a person says they are.

I to wouldn't consider 4.11 a good ratio for 35s, I also know little xj brakes aren't going to be braking monster's .

Make sure no leaks, no air, good parts and don't drive it over your head know its limits and capabilities and don't expect it to perform outside those parameters.

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[Tim_MN]

Are all the soft brakes hoses braided steel ?

 

[RCP Phx]

Manifold vacuum is highest when idling and low rpm's, what I would take a look at is the check valve. Check your rotors for end play and run-out. Also check your wheel bearings for wear and proper adjustment.

 

[jmg222]

Youl likely not see any improvement with swapping to disc rear.

My thinking exactly.

One if my first thoughts is that there's air in the system. Seems there always is no matter how badass a brake bleeder a person says they are.

On my list to double check.

I to wouldn't consider 4.11 a good ratio for 35s, I also know little xj brakes aren't going to be braking monster's .

Why aren't 4.11's a good ratio for 35s?
Also, I'm not running stock brakes , I'm running vancos up front, stock ford 8.8. drums in the rear,

 

[RCP Phx]

You might also look at your pads, I find that the EBC Yellow Truck pads work with no warming up and even better when hot.

 

[Evan03]

I have no idea what black magic and Vancouver are but I know if you bolted them on theyre similar sized to stock and will see little or no improvement over normal pads
Youl get better brakes from more clamping area and force not fancy names.

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[RCP Phx]

For the un-educated.......
https://www.shop.blackmagicbrakes.com/VANCO-15-Big-Brake-Kit-with-BMB-Pads-for-15-Rims-BBK-NDM15.htm

 

[jmg222]

I have no idea what black magic and Vancouver are but I know if you bolted them on theyre similar sized to stock and will see little or no improvement over normal pads
Youl get better brakes from more clamping area and force not fancy names.

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Google 'em. They're pretty well-known upgrade for jeep/xj front brakes.
Still wondering what the issue is with 4.11 gears/35s on my truck?

 

[Evan03]

Better than stock. 3.55-4.46 is 1 gear change. 4.11 about half a gear not realy worth it for about any tire size over 31

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[Evan03]

1200 bucks and still shitty brakes? Glad im uneducated and only spent pennies on my brakes to get the same results

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[ljobbins]

This is what I see, in your signature it says you swapped in a motor with way more horsepower than the stock 4.0. And with an automatic transmission, the engine/transmission is always trying to push the car when it’s in gear. Combine that with large tires and you are way past the designed capability of the small stock brakes. You would need larger brakes to match the tire and horsepower/torque of your car. Compare the size of tires and brakes for other cars with a lot of horsepower and torque and they all run mallee ties or larger axles with larger brakes.

 

[jmg222]

Better than stock. 3.55-4.46 is 1 gear change. 4.11 about half a gear not realy worth it for about any tire size over 31

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keep in mind I'm running a chevy zz6 motor and a 700r4. I was actually thinking of lowering the ratios. But anyway, to each their own.

 

[jmg222]

This is what I see, in your signature it says you swapped in a motor with way more horsepower than the stock 4.0. And with an automatic transmission, the engine/transmission is always trying to push the car when it’s in gear. Combine that with large tires and you are way past the designed capability of the small stock brakes. You would need larger brakes to match the tire and horsepower/torque of your car. Compare the size of tires and brakes for other cars with a lot of horsepower and torque and they all run mallee ties or larger axles with larger brakes.

That's kind of where I'm zoning in on. probably time for larger wheels to fit larger brakes...