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Discussion Starter #1
A while ago, my buddy Tom gave me a Brake booster and master cylinder that is an upgrade from the Single diaphram brake booster that is in the older (up to 94 I think) XJ's

The new style is a dual diaphram brake booster instead of a single diaphram and the ones that will work are 96+ XJ's and 99-2004 Grand Cherokees. I've heard it makes quite a bit of difference in how well the brakes work, and after my axle swap, my brakes feel pretty weak. So I hope this helps.

There's a difference in the size of the hole that is in the push rod that hooks up to the brake pedal and in the way the brake light switch will operate with that push rod. One fix is to drill the hole larger in the new boosters rod, and to grind the back edge flat so that the brake light switch will operate properly.

The one on the right is the new one

The new master cylinder is smaller and has metric fittings for the brake lines, where as the old M/C has standard fittings and they are on the other side of the M/C. I'll have to do some work to get those hooked up.

I have the booster installed and the new master cylinder setting on it. The nuts that bolt on that master are different size than the old ones. I need 8mm instead of the old 3/8.

I rounded up the brake line parts I needed and got it all hooked up.

There are two 3/16" brake line tubes, 12" long, that are just barely long enough.

Then I used a total of four adapter fittings, I went to NAPA and they fixed me right up.

There is....
a 3/16" female to 1/4" male adapter
a 3/16" female to 5/16" male adapter
a 3/16" female to M12 x 1.0 male bubble end adapter
a 3/16" famale to M10 x 1.0 male bubble end adapter

I don't know which adapter part number is which, but the four numbers are..


The washer bottle just barely will fit now, the circles show where it is against the booster and against the air filter box. I put some spacers under the mount points to raise it about 3/4" and moved it forward about an inch.

I went ahead and drilled out the new push rod with a stepped drill bit to 5/8", then I ground a flat spot on the end that looks just like the old push rod. The brake light switch seems to work fine.

The pedal is higher now and that takes a little getting used to, I guess that's common to this upgarde, but....

Man what a huge difference this makes

The pedal is much higher and it stops so much better, it's amazing!

I used to sort of anticipate stops and pump the pedal twice, and that helped stop it but it still wouldn't do well under hard braking, and a panic stop would have been real scary.

Now the first pump is real high and firm and it stops better then when it had the stock tires on it.

I am so glad I did this upgrade and....

After driving the Xj in Moab for a few days, I'm more impressed then ever with this upgrade.

The brake pedal was always right there and firm, it stops the truck great going down steep hills and obstacles.

I highly recommend this to anyone with a single diaphram booster.

23 Posts
I will do it!.. but, is there a solution for the higher pedal?...

59 Posts
I will do it!.. but, is there a solution for the higher pedal?...

Simple fix, the rod on the newer style is longer. With the old and new side by side measure from the back where the 4 studs are that attach to the firewall to the center of the hole on the old one.
Then measure the new style and see how much longer it is to the center of its hole. Take a cut off wheel and cut off the mounting hole from the end of the rod leaving that piece intact. (ie: cut it off where the rod meets the outer part of the hole mount)
Then subtract the distance that the new rod was longer than the old and cut that much off the end of the rod...... if the new style rod is 1/2" longer, cut out 1/2"
Have someone wire weld the hole mount back to the end of the rod making sure to keep wet rags on the rod to keep any heat from getting to the diaphrams.
Its a easy fix, 10 minutes tops to do if you have a cut off tool and a welder. We do that all the time when upgrading classic cars to newer brake setups.
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