Can't lock front brakes even after BBK install - JeepForum.com
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post #1 of 35 Old 06-09-2018, 04:14 AM Thread Starter
glook
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Can't lock front brakes even after BBK install

I've been driving with not enough brakes to lock my 35" tires for quite some time and almost got used to it. I swapped my rear brakes from drums to discs and that changed nothing except the ease of maintenance. Then I swapped my 2.5+AX5 to a 5.3+4L60E and immediately found out that I desperately need better brakes. Not wanting to redo my steering and track bar, I bought a 15" big brake kit from black magic brakes. Installed it, replaced all my brake hoses, did the break-in.

Now, the problem. I still can't lock my front brakes on clean dry asphalt. I thought that I didn't bleed the brakes enough and tried bleeding them again, but putting 2 more liters of brake fluid through the system didn't help. The pedal still feels a bit softer than before the BBK install.

Another issue I have is that I added some preload to the brake prop valve spring, a total of 1/8" of washers. But I can't get my rear brakes to lock up even on sand. I tried putting a wire to the valve to check that its not stuck. It moves by about 1/16", so the valve is not stuck.

I am at a loss here. Can a bad master cylinder cause this? Or maybe I have a bad vacuum booster? Anything else I might be missing?

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post #2 of 35 Old 06-09-2018, 05:22 AM
hokahay
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You lose control of the vehicle when brakes lock up, because you lose steerage. It's not a good thing, nor is it desirable.
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post #3 of 35 Old 06-09-2018, 05:52 AM
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While not desirable, the ability of the brakes to lock the tires is a way to evaluate their effectiveness.

You mention not being able to lock your rear brakes. I don't think that is unusual due to weight transfer when braking, disc brakes don't affect this. You should be able to lock the fronts, however.

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post #4 of 35 Old 06-09-2018, 06:18 AM
jay-h
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The 35 tires would much harder to lock than standard size tires. Your braking is still effective, but locking is unlikely

Brake effectiveness is all about the amount of energy absorbed... which you have obviously improved greatly with the new calipers regardless of locking.


The next step after bigger brakes is a bigger booster to increase the contact area pressure-- but personally, I wouldn't sweat it.

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post #5 of 35 Old 06-09-2018, 06:23 AM
biffgnar
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Can you confirm that the calipers are on the correct side?
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post #6 of 35 Old 06-09-2018, 06:53 AM
mrblaine
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Quote:
Originally Posted by glook View Post
I've been driving with not enough brakes to lock my 35" tires for quite some time and almost got used to it. I swapped my rear brakes from drums to discs and that changed nothing except the ease of maintenance. Then I swapped my 2.5+AX5 to a 5.3+4L60E and immediately found out that I desperately need better brakes. Not wanting to redo my steering and track bar, I bought a 15" big brake kit from black magic brakes. Installed it, replaced all my brake hoses, did the break-in.

Now, the problem. I still can't lock my front brakes on clean dry asphalt. I thought that I didn't bleed the brakes enough and tried bleeding them again, but putting 2 more liters of brake fluid through the system didn't help. The pedal still feels a bit softer than before the BBK install.

Another issue I have is that I added some preload to the brake prop valve spring, a total of 1/8" of washers. But I can't get my rear brakes to lock up even on sand. I tried putting a wire to the valve to check that its not stuck. It moves by about 1/16", so the valve is not stuck.

I am at a loss here. Can a bad master cylinder cause this? Or maybe I have a bad vacuum booster? Anything else I might be missing?
Can you post a picture of the wear pattern on the rotor after driving it and then a picture of how the caliper is mounted?

That and stop trying to get the rears to lock up even in the dirt.
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post #7 of 35 Old 06-09-2018, 06:55 AM
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Originally Posted by hokahay View Post
You lose control of the vehicle when brakes lock up, because you lose steerage. It's not a good thing, nor is it desirable.
If you can't lock them up, how do you know how far away you are from being able to?
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post #8 of 35 Old 06-09-2018, 06:59 AM
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Originally Posted by mrblaine View Post
If you can't lock them up, how do you know how far away you are from being able to?
MrBlaine, I certainly defer to your brake expertise.
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post #9 of 35 Old 06-09-2018, 07:48 AM Thread Starter
glook
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Quote:
Originally Posted by mrblaine View Post
Can you post a picture of the wear pattern on the rotor after driving it and then a picture of how the caliper is mounted?
I have attached a picture of the wear pattern and a picture of the passenger side caliper.

Quote:
That and stop trying to get the rears to lock up even in the dirt.
I'm not trying to get them to lock up, it just concerns me that other people add 1/8" spacers to the prop valve and get the rears to lock up on dry pavement and in my case the result is so much different.
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post #10 of 35 Old 06-09-2018, 08:00 AM
mrblaine
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Originally Posted by glook View Post
I have attached a picture of the wear pattern and a picture of the passenger side caliper.


I'm not trying to get them to lock up, it just concerns me that other people add 1/8" spacers to the prop valve and get the rears to lock up on dry pavement and in my case the result is so much different.
Go do the break in again more aggressively.

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post #11 of 35 Old 06-09-2018, 10:00 AM
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Quote:
Originally Posted by mrblaine View Post
Go do the break in again more aggressively.
X2, they'll get hot enough that you'll be able to smell them during the break-in process when done properly.

I installed my BBK 4 years ago and periodically brake them aggressively so they continue to brake well. About a month ago I did another short break-in cycle & tested them afterward and they're still easily locked up. They won't brake nearly as well if you don't do the break-in aggressively/properly as per the instructions. And after getting them broken in properly so they brake up to their potential, you can't baby them or they won't continue working as well.
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post #12 of 35 Old 06-09-2018, 11:07 AM
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With a setup like that, you should be able to lock them up.
I'm totally stock except for black magic pads with centric rotors, it'll lock up my 33's.

During break in, go out and put your leg muscle into it, as hard & fast as you can
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post #13 of 35 Old 06-09-2018, 12:55 PM Thread Starter
glook
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Quote:
Originally Posted by mrblaine View Post
Go do the break in again more aggressively.
Ok, I must admit that I underestimated the importance of the break-in process.

I did two series of aggressive stops from 55 mph today. During the second series I unintentionally locked my front brakes at 40 mph. That's a lot of tire smoke Stopping power changed drastically between the first and the last stop of the series. If the performance stays at this level, that'll be great. I just hope I won't have to warm up the pads each day.

I'll report on braking performance after I do another two series of stops tomorrow. Two questions remain, however.

Brake pedal is softer now than it was before the BBK swap. Is that to be expected? I liked the firm pedal feel that I had before.

And the second one is about the rear brakes. Is the fact that they don't lock up on sand even after adding preload to the prop valve spring an indication of a trouble somewhere? Or maybe the spring just sagged over the years?

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post #14 of 35 Old 06-09-2018, 01:38 PM
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They shouldn't need warmed up if always used aggressively. If babied performance will fade and you need to do break in again.
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post #15 of 35 Old 06-09-2018, 01:55 PM
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Quote:
Originally Posted by glook View Post
And the second one is about the rear brakes. Is the fact that they don't lock up on sand even after adding preload to the prop valve spring an indication of a trouble somewhere? Or maybe the spring just sagged over the years?
Your braking system was designed to help prevent the rear brakes from locking up, which is the purpose of the proportioning valve. Locked rear brakes can throw a vehicle into a skid or out of control.
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