I was getting a hissing sound every time I pressed the brake pedal, combined with progressive loss of power assistance (normal assistance when the pedal was first pressed, but dropping off quickly, meaning you had to be standing on the pedal by the end of the stop), which pretty much pointed to the brake booster. Picked up a new OEM one and swapped it in today, using a mix of the FSM and Haynes for guidance.
There donít seem to be any how-tos for the KJ brake booster, so here we goÖ
Remove the battery. This isnít part of the FSM procedure, but it greatly eases access and gives you some essential (IMO) extra working room.
Disconnect the large harness connector above and to the left of the booster. This stops the wiring interfering with the booster when you remove/replace it.
Disconnect the electrical connector for the brake fluid level and the two brake lines into the master cylinder. You will need a 12mm flare nut wrench. I have both a standard flare nut wrench and a set of 3/8 drive crowsfoot flare nut sockets I picked up cheaply from a local tool shop.
I used the 12mm crowsfoot with my breaker bar to get each tube nut started, which made life much less strenuous. I then switched to the standard FN wrench to finish off.
Remove the two master cylinder mounting nuts. They are both 13mm. I needed a wobble extension to get onto the lower nut because it was slightly obstructed by brake lines.
Remove the master cylinder assembly and set aside. I put it inside a plastic bag for protection and to catch any fluid leaks.
The FSM just says ďremove any obstructing brake linesĒ. For me, I had to remove all the indicated lines (12mm flare nut socket again):
Remove the two 10mm nuts from the HCU mounting bracket (location roughly indicated above). Carefully lift the bracket off the studs and move it aside. I thought I might have to remove the last remaining brake line from the top of the HCU, but it wasnít necessary, there was just enough play for me to move it as far as I needed.
Remove the trim panel from the driverís knee area by putting your hands into the cutout under the steering column and pulling to the rear. Be forceful but careful! The clips should disengage and the panel will hinge down. Slide it to the left and remove it completely.
Remove the two Phillips screws holding the black foamy noise insulation panel. You donít need to remove this panel completely, once the screws are out youíll have the access you need.
Remove the clip securing the booster pushrod to the brake pedal. I had to pry a tang up away from the head of the attaching pin before the clip would pull off. Then remove the pushrod from the pedal.
The booster is attached with two nuts. You will need a 13mm deep socket.
Once the nuts are out, you should be able to finagle the booster out of the engine bay.... and get the new one in
Tighten the mounting nuts, reattach the pushrod and replace the clip.
If youíre replacing the booster, you have to replace the brake light switch as well. According to the FSM and Haynes, you turn the switch counterclockwise about 30 degrees and pull it out. After some minutes of cursing, I discovered that on my KJ, you have to turn it clockwise. Once itís out, disconnect the harness connector and throw the switch away (it has DO NOT REINSTALL stamped onto it).
Connect the harness connector to the new switch and install it according to the instruction sheet included with it. Youíre now done under the dash.
The remainder of the job is, as always, the reverse of removal. I didnít bother buying a new check valve, I just swapped the old one onto the new booster. It needed some dish soap to persuade it into place.
Once everythingís buttoned back up, youíll have to bleed the master cylinder, and then the lines coming out of the top of the HCU, and then finally the wheels, in the sequence rear right, rear left, front right, front left.
Factory procedure is to follow the manual bleed with a DRB3 bleed of the ABS, and then a final manual wheel bleed. Since I donít have a DRB3 and the only place nearby that does is the stealership, I acted on a suggestion from streetglideok; I drove the Jeep to a nearby sand lot and had some fun triggering about 4-5 ABS stops.
The idea is that if thereís any air trapped in the HCU, the ABS stops will cause it to be pumped into the brake lines and youíll feel it as a spongy pedal, in which case you need to do another manual bleed. The brakes felt normal on the way to the sand lot and they felt normal on the way back, so Iíll call that good.
Hope that all helps somebody!