Dual Diaphragm TJ Brake Booster Upgrade - JeepForum.com
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post #1 of 21 Old 06-09-2020, 07:24 AM Thread Starter
daddyjeep87
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Dual Diaphragm TJ Brake Booster Upgrade

When we put bigger tires and lower gears on our Jeeps we find out pretty quickly that our brakes could be a little stronger. If you have done any research on the topic you will find that in '95 they put dual diaphragm boosters on YJs with the 4.0. These boosters have proven to many to be a little difficult to track down since they were a one year only thing and most parts guys don't know the difference. Enter plan "b"... The prevalent TJ. All Jeep TJs use a dual diaphragm booster. I also understand that Cherokees and Grand Cherokees do as well, but I haven't confirmed that. The TJ booster is almost the same, but the required mods are pretty easy.

Stock YJ single diaphragm booster...



OK, let's look at why the TJ is different that the YJ. First off we can see that the rod that connects to the brake pedal is much longer on the YJ booster. You can also see that the TJ booster is much thicker. The mounting studs on the TJ booster are a little bigger too. That's it.

YJ booster in the front and TJ in the back...



So first I am going to address the length of the connecting rod. I have read where most people make this a little more complicated by adding a threaded collar and making it adjustable. I really didn't see the need for all that. I simply measured from the mounting surface to the center of the hole and got 7 9/16". No fancy measuring device needed here. I just used a tape measure. Lets examine what happens if your are off a little bit here.... not much. The only thing that would be off it the height of the brake pedal from the floor. A reasonable accurate measurement here is just fine.

I cut off the rod pretty close to the YJ booster to be sure I had enough rod to work with. Then I held it up to the TJ booster and lined it up with my tape measure as best I could and made a mark on the TJ shaft where I cut it. Sorry no pic here. I barely had enough hands to perform the task.

Once you verify that your cuts were good add a nice bevel to both rods...



Here is where people are going to argue with you and bring up the driving into a bus load of nuns or something. I just welded the two together. Lets look at the forces that are applied to this rod... when you push on the brake pedal you push the rod forward to activate the brakes. This is very linear and puts almost no stress on the welded joint. When you release the brake, the spring inside the booster pushes it back putting even less stress on the weld. There is no situation where there is any lateral or even pulling force put on your weld. Not much to worry about here.

All welded together and ground smooth...



Next difference to address, the mounting studs are bigger. I drilled out the 4 holes in the firewall mount to 3/8". That's it... done.

Final difference, the booster is just plain bigger. This may affect how everything lines up to your brake lines. The proportioning valve is identical between the TJ and the YJ. My YJ lines bolted up to the TJ booster just fine. I did relocate the proportioning valve to the YJ location instead of the TJ location. The TJ proportioning valve was bolted to the mounting studs for the master cylinder. My lines wouldn't reach so I made some new lines to move the proportioning valve down to where I needed it. I tried to replace all of my fitting, but that proved to be nearly impossible. My Napa didn't have anything that would work so I just reused them all.

IMPORTANT!!!! The connection to the master cylinder uses a bubble flare. The connection to the proportioning valve uses a double flare. I learned this trick a while back from Ian Johnson. When tightening your fitting with new flares, tighten it until it is snug, loosen it a bit, tighten a little more, loosen it again, and repeat this a few times for each fitting. It works much better that just trying to tighten it real tight as it forms the flare to match the seat.

Obviously you will need to bleed your brakes real good when you get it all back together.

Here is is all put back together...



The brakes do feel better in the shop. We will know more when we get it out on the trail and I will report back.

This whole thing can be done in a couple of hours. I spread it out over a couple days because I didn't have a lot of time. It was surprisingly simple.

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post #2 of 21 Old 06-09-2020, 07:29 AM
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Nice work, thx for the how-to.


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post #3 of 21 Old 06-09-2020, 07:49 AM
JeeperDon
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I had no problem getting a 95 dual a couple years ago. The local O'Reilly's had it in stock. I returned my single as the core. Even with that, I recall I did have to mod the spacer bracket between it and the firewall.

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post #4 of 21 Old 06-09-2020, 08:00 AM
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Nice.

I like layers of threads and how to's on a topic/mod.

Have the TJ booster sitting in my parts storage. Waiting until I retire the jeep to do this so I dont encounter any issues while needing the jeep on the road.

Thanks again man.

~YJOTM MAY '16, JULY '19~


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post #5 of 21 Old 06-10-2020, 06:46 AM Thread Starter
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The next piece of this is getting stronger rear brakes. Just rolling a couple feet in the shop and mashing the brakes the front just slides. I really can't tell if the rears are doing anything. I had the Jeep on the lift a while back and ran it in the air and I couldn't hold the rear wheels from turning int the air. The damn Eldorado calipers may have something to do with it. So in an effort to get stronger rear brakes I am going to modify the proportioning valve. Lots of people think this is dangerous on the road, but I don't care as my Jeep isn't built for the road.

This is really easy.

Remove the bolt on the end of the valve. It has a spring behind it so make sure you hang on to it. Set the bolt and the spring aside.



Pull out the shuttle valve. This will require some needle nose pliers or hemostats.



Remove the black plastic thing on the shuttle valve. Some refer to this as an o ring, but it is not. Whatever you want to call it, get rid of it.

reinstall the shuttle valve, spring, and bolt.



Bleed your brakes...again. I have read that only the rear brakes will need blead, but just do yourself a favor and do all 4. It doesn't take that long. Especially if you have a vacuum bleeder. If you don't, go get one

87 YJ, 5.7/TH350/NP241/HP Dana44/9", Custom 4 link/ Radius Arms, ADS Nitrogen Shocks, 35" Cooper STT


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post #6 of 21 Old 06-10-2020, 07:35 AM
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One thing worth noting... I did have trouble finding a YJ combo. However, I didn't have a problem finding a TJ combo. I did have a problem finding a TJ combo for a reasonable price though. At the time I could get a used WJ setup for under $50 shipped all day long. Looks like TJ setups are similarly priced now but when I looked a year or two ago, they were at least double the price. Either setup will be a similar mod and definitely worth it.

On a side note... With a D44 and a Ford 9", does your pedal not sink to the floor with almost no effort? That was the problem I was having with my D44/8.8 and had to upgrade to a bigger MC bore to get rear brakes back.


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post #7 of 21 Old 06-10-2020, 08:37 AM Thread Starter
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Quote:
Originally Posted by Waternut View Post

On a side note... With a D44 and a Ford 9", does your pedal not sink to the floor with almost no effort? That was the problem I was having with my D44/8.8 and had to upgrade to a bigger MC bore to get rear brakes back.
Currently... yes it does sink to the floor. Of course that is due to the leaking Eldorado caliper. I do have a nice firm pedal, it just creeps to the floor due to the leak. Hopefully I can get everything worked out tonight.

87 YJ, 5.7/TH350/NP241/HP Dana44/9", Custom 4 link/ Radius Arms, ADS Nitrogen Shocks, 35" Cooper STT


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post #8 of 21 Old 06-16-2020, 08:03 AM Thread Starter
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UPDATE......

I couldn't get that damn Eldorado Caliper to stop leaking so I ordered up a '79 K20 caliper. I got my Part Friday Morning and planned on installing it, bleeding the brakes, and loading DaddyJeep on the trailer and going to Rausch Creek for the weekend. Wouldn't you know it...The caliper is too big. Come to find out that I don't have the Eldorado Calipers that I thought I did. I have the 80's metric caliper. They are considerably smaller. They have a 5 1/2" mounting width vs the 7 1/2" that the 76-77 calipers do. Now What??? Well, I am certainly not cancelling my wheeling trip. I shoved a bolt through the fitting where the banjo bolt goes with an o ring on both sides and loaded it up with three brakes. My buddy was driving so That gave me some time to sort things out and I learned that I needed a caliper from an 80's metric car... like an '80 Malibu. I called ahead to the closest Autozone to where we were headed and they just happened to have the correct caliper and some new pads. I installed the new caliper and pads on the trailer and gravity blead both rear brakes for a bit.

I rolled it off the trailer and did a couple hard stops in the gravel parking lot. The front brakes lock up in the gravel with ease and the rears lock up with a bit more pedal pressure. This is absolutely perfect. Those that are skeptical about the modification to the proportioning valve are not correct in my case... NOTE... your results may vary. This works perfectly with MY Jeep. Weight distribution, different axle and brake configurations, and a host of other variables will change this. Do your own testing. That solves my brake bias portion of things.

Now, for my overall brake performance. My brakes pedal is a bit soft. I most likely need to bleed the brakes better. I am going to go ahead and get rid of the other Eldorado caliper so I will do a good bleed then. Even with the soft pedal my brakes are much, much better. The pedal force needed in 4 low is so minimal now. No more leg cramps... no more sore foot.

This is a fantastic upgrade to my braking. I did some gnarly descents dropping off some 4' ledges and navigating boulder fields while descending a 30 degree grade with confidence.

87 YJ, 5.7/TH350/NP241/HP Dana44/9", Custom 4 link/ Radius Arms, ADS Nitrogen Shocks, 35" Cooper STT


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post #9 of 21 Old 06-16-2020, 12:16 PM
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Try to bleed one more time really well. If the brake pedal is still soft, you may be fine but know that ultimately, the MC bore is too small for your brakes and you aren't getting the full braking force from your calipers.

Long descents kinda suck with a soft pedal. Really wears your leg out. Yes, I know engine braking is the way to go but I often find that one gear is way too slow and the next is way too fast for whatever group I happen to be in. The other problem I have is with a V8 and automatic trans, low range and a steep hill requires the full pedal stroke which I'm not a fan of and I have a larger MC.


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post #10 of 21 Old 06-16-2020, 01:03 PM Thread Starter
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Quote:
Originally Posted by Waternut View Post
Try to bleed one more time really well. If the brake pedal is still soft, you may be fine but know that ultimately, the MC bore is too small for your brakes and you aren't getting the full braking force from your calipers.

Long descents kinda suck with a soft pedal. Really wears your leg out. Yes, I know engine braking is the way to go but I often find that one gear is way too slow and the next is way too fast for whatever group I happen to be in. The other problem I have is with a V8 and automatic trans, low range and a steep hill requires the full pedal stroke which I'm not a fan of and I have a larger MC.
Even with a softer than preferred pedal the brakes are a huge improvement. They were a joy to hold on technical descents vs what they were before.
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post #11 of 21 Old 06-16-2020, 01:27 PM
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Quote:
Originally Posted by daddyjeep87 View Post
Even with a softer than preferred pedal the brakes are a huge improvement. They were a joy to hold on technical descents vs what they were before.
Funny because someone hopped in my Jeep the other weekend and said "wow is that what good brakes are supposed to feel like?". Meanwhile, I'm over here thinking, "my brakes still aren't perfect, I wish they were better".
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post #12 of 21 Old 06-17-2020, 06:46 AM
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So I have a 94 YJ. If I pick up a 95 brake booster will my 94 master cylinder bolt to the new brake booster?

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post #13 of 21 Old 06-17-2020, 06:59 AM
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Quote:
Originally Posted by NotmyJK View Post
So I have a 94 YJ. If I pick up a 95 brake booster will my 94 master cylinder bolt to the new brake booster?
If you have the metal, 2 chamber master cylinder then you will definitely need the newer one with the plastic reservoir. They are grossly different.
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post #14 of 21 Old 06-17-2020, 07:01 AM Thread Starter
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Originally Posted by NotmyJK View Post
So I have a 94 YJ. If I pick up a 95 brake booster will my 94 master cylinder bolt to the new brake booster?
I'm not sure.

87 YJ, 5.7/TH350/NP241/HP Dana44/9", Custom 4 link/ Radius Arms, ADS Nitrogen Shocks, 35" Cooper STT


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post #15 of 21 Old 06-17-2020, 07:07 AM
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Quote:
Originally Posted by Waternut View Post
If you have the metal, 2 chamber master cylinder then you will definitely need the newer one with the plastic reservoir. They are grossly different.
Yup, I have the metal reservior master cylinder

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