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Back on the other side of the firewall, we have a couple of things to do before the factory booster and master cylinder can be removed. Remove the lines that connect the master cylinder to the combination valve. Grab some paper towels as you will have brake fluid dripping here and there. The mounting bracket for the proportioning valve is mounted to the studs that hold the master cylinder and booster together. Carefully remove the two nuts and then slip the mounting bracket off the studs. As I wanted the booster and master cylinder to remain as one piece once I removed it from the TJ, I put the nuts back on and snugged them down.
Also note there is a rather large vacuum hose (the vacuum source that was attached to the booster) that I have disconnected in the above pic. You need to seal off the hose so that you do not suck air into your intake manifold and lean out the engine (which can overheat it) or get one of those rubber caps that fit over the fitting on the intake manifold to seal it. I folded the hose in half (causing the hose to collapse on itself) and then used a couple of big zip ties to keep it squeezed together. A bolt, hosed clamped into the hose, would work equally well and is what I'll do once I find the right bolt in highly unorganized bolt collection. If you are looking at the above combination valve and you notice it does not look like yours, not to worry. It is from a ZJ and was installed after I added disc brakes to the rear axle.
With all the above items disconnected, have a friend remove those 4 nuts from the other side of the firewall while you hold the booster and master cylinder. Once the nuts are removed, carefully extract the entire unit from the hole in the firewall. There will be a rubber gasket stuck on the firewall (more than likely) that covered the pedal rod going through the firewall. Remove it as it is no longer needed.
You will notice that the mounting studs that hold the hydraulic booster to the master cylinder are about a 3/16"" further apart than the holes on your proportioning valve's mounting bracket. (that is true for the ZJ mounting bracket which I believe has the same dimensions as the TJ) I grabbed a cordless drill and enlarged the holes to 3/8". This took care of some of the problem but they still didn't quite fit. With a Dremel tool, I cut a small piece out of the end of the mounting bracket so I could spread the far end of the bracket open, much like a those U shaped electrical terminals. It took just a couple of minutes to open up the bracket enough so that it would fit over the mounting studs. Once the nuts were tightened back down, you could not tell any handiwork was done.
It was time to mount the booster (and master cylinder) to the firewall. This time, it is your turn to lay on your side to get up and under the dash. Have your friend carefully put the booster rod through the large opening and line up the mounting studs (two of them are pressed into place and two are loose, although it is possible that Van may be using regular bolts in future kits) with the corresponding holes in the firewall. You will also notice about now that the overall length of the booster and master cylinder is an inch or two longer than what you just took out. (yes, I measured mine before I started the install and realized that I could make it fit with just a little alteration) I think it is called the purge solenoid and is mounted to the side of the rather large canister that bolts to the fender well, right next to the master cylinder. The purge solenoid and its myriad of small noses was slipped off of its mounting tab and the tab cut off with the Dremel tool. That provided adequate clearance for the master cylinder to fit where it needed to be. I used two zip ties to reattach the purge solenoid to the canister bracket. A small bracket made from light gauge metal could be fabricated and the purge valve remounted if so desired.