From left to right: '89 MJ, '97 XJ, '00 WJ
We spend a lot of time in junkyards. Usually we are hoping to stumble across a front Dana 60 or an LS1-powered car. We've yet to find either of them, but we did stumble across a cheap and easy way to upgrade the braking systems on pre-'95 Cherokees and Comanches.
From '84-'94, the Cherokee and Comanche were stuck with a single-diaphragm brake booster. It worked OK for the stock application, but once a lift and tires factored in, the inadequacies of the stock parts became apparent. After driving a few late-model Cherokees and noticing how much better the brakes are, we decided to swap the late-model vacuum booster and master cylinder into an early-model Jeep.
So when we found a '97 Cherokee in the boneyard, we pulled the parts we thought we wanted for the swap. However, once we got the booster and master home and compared them to the stock booster and master out of our '89 MJ, we saw that the pushrod was too long. After some more research, we discovered that the '99-'04 Grand Cherokee booster and master had a shorter rod and would make for an easy swap.
How it works is the later Cherokee and Grand Cherokee use dual-diaphragm boosters which increase the line pressure in the brake system, apply more pressure to the brakes, and make you stop faster.
What we didn't get right off the bat was just how easy it would be. From start to finish, it took us right around three hours to almost double our braking. We didn't measure distances or anything, but with no ABS, we went from only being able to lock the brakes up in the dirt to being able to lock them up anywhere. Now we feel safe enough with the brakes to add bigger tires.
Interchange Index
There are lots of ways to go at this swap. If you've got an early XJ or MJ, the easiest way would be to try to find a '95 or '96 setup, and swap it in with no spacers or rods to swap. Next up is the Grand Cherokee route. We used WJ parts, but ZJ parts also work; however, we've heard the WJ booster and master provide stronger brakes. The last route would be to use the '97-'01 Cherokee parts and send the booster out to be rebuilt with the stock rod that was in your Jeep to begin with.
This is the late-model XJ booster, with its 1/4-inch spacer beyond. Without the spacer, the center of the hole in the rod is 5 3/4 inches from the back of the booster. The stock MJ rod was at 5 inches while the WJ measurement was 5 3/16 inches. | 
The '90-and-earlier XJs and MJs have a goofy brake light switch. Rather than actuate the brake lights directly off the brake pedal, it senses pressure on the bolt that goes through the brake pedal to light them up. Because of this, we had a couple of extra steps that you '91-'94 guys won't have to worry about. |
The distance from the center of the hole to the edge where the switch mounts is critical. After drilling the hole out to 5/8 inch, we used the drill bit to center the cut-off piece of the stock booster, clamped it down, and carefully ground the new rod to match the old. | 
We made up the 3/16-inch difference between the back of the booster and the center of the hole in the pushrod by cutting and drilling a 3/16-inch aluminum plate to fit. This measurement is important, because too short of a rod and you won't get the full travel from the brake pedal, while too long of a rod means that the brakes will always be partially applied. We then put weatherstripping on it to keep the hot engine-compartment air out of the cab. |
Make your life easier and grab the lines to the proportioning valve out of whatever you yank the other parts from. The WJ uses a metric bubble style fitting (left), while the '89 MJ uses a standard double flare (right). We simply cut the two lines, put the '89 nuts on the late-model lines, and reflared it so the lines would bolt into the stock proportioning valve just like they were supposed to be there. | 
We had to bend the pinch seam up to clear the bigger booster even with the 3/16-inch spacer. Otherwise, our washer fluid reservoir and air filter fit right back in the stock locations. Sure it's a tight fit, but nothing rubs, and after bleeding the brakes, we went from worrying about hitting people who cut us off on the freeway to worrying about getting rear ended when we have to panic stop. |
wordgoeshere
If you don't have the time to spend crawling around in junkyards, as of press time, Crown Automotive has the two major parts you need to do this swap: the WJ booster (PN 5011261AB) and the WJ master cylinder (5011260AA). They are OE-quality and come with a warranty. Some junkyards just want too much money for the parts and then want you to pull them.
Also as of press time, Crown is working on putting together a complete kit to make this swap work for you. The company is still up in the air about what exactly the kit will entail, but the intent is to make an already simple swap easier. So if you don't want to hunt down the parts on your own, call Crown Automotive first to see what it has put together.