EarlyCJ5.com Technical Library

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Dual Chamber Master Cylinder Conversion

By: Dennis Davie

Edited for the web by: Adam Sparks

The dual chamber master cylinder conversion kit offers safety and peace of mind for those of us that wheel classic JEEPs. Herm the Overdrive Guy sells the kit for a fair price and he is easy to deal with. The kit can be found here: www.hermtheoverdriveguy.com.

First off, let me tell you this is a major project for the back yard JEEP builder. Due to trial and error, it has taken me several days to complete this project. On a scale of 1 through 10 (10 being the most difficult), I'd give this project a solid 8.5. It would be much easier with the tub off.

Tools you'll need:

The old master cylinder needs to be removed first. Remove the brake lines and the stop light switch while the old M/C is still mounted to the frame. Next, remove the cotter pin that holds the M/C support bracket on to the clutch shaft tube. Once that is done, you'll be able to unbolt the old M/C from the frame. Be sure to save the rubber boot from your old master cylinder. You'll need it later.

The next step is to remove the rivets from the clutch shaft tube support bracket. If you are lucky, there will be bolts instead of rivets. Just unbolt and move to the next step. I used the air chisel to pop the heads off of the bottom of the rivets. Once that is done, take a wire wheel to the support bracket. Be sure you see the perfectly round circles where the rivets are. If they are not round, you will have to grind until they are. Once round, I used a nail set to punch a starting point for my drill. I used a unibit and drilled until I had the bottom part of the rivet shaft removed. Take a center punch and finish removing the rivet. Now do the next one.

There will most likely be a reinforcement plate on the frame rail just above the clutch tube support bracket. The instructions you will have with the kit will tell you to remove it for clearance issues. I was fortunate and the new bracket fit right into place without removing this plate.

You will now have to remove the old M/C mount from the frame. You will need a die grinder for this if your wiring still runs along the frame rail (I would not use a torch here). This is a time consuming process while lying on your back, but it must be done correctly. Completely remove all of the metal from the frame. You will have to take 1/8" off of the top frame rail as well. Do not make the mistake of not doing this. It'll give you more grief than you can imagine.

Remove the plunger rod from the brake pedal shaft and dry fit the new bracket and M/C into the frame rails. Be sure you can remove the M/C cover easily. If not, take some more metal out of the frame rail. Next, check to be sure that the brake pedal arm doesn't touch the bracket half way down. Mine did, and I had to cut a notch into it.

Also take note as to the relationship of the transmission bellhousing to the brake line outlet ports on the new M/C. I didn't have much clearance for the front brake line port, but I am running a SBC so you may have enough room.

Make sure the plunger rod is the proper length. It should be seated properly into the piston of the new M/C, and checked against the brake pedal shaft. Take your china marker (or grease pencil) and mark the position of the nub that pushes the plunger rod. My plunger rod was exactly 1.5" too long. Now, unbolt the new M/C and bracket and use your angle grinder to soften the sharp edges on the frame. Note: I needed to remove the brake pedal arm from the clutch tube support shaft to be able to bolt the new M/C to the new bracket.

Unbolt the M/C from the new bracket and remove the 3/16" plug from the bottom. This is where I mounted the brake light switch. It works real nice there. Now you can bench bleed the M/C.

Adjust your plunger rod to the proper length. If there is not enough adjustment to shorten (as in my case), you'll need to add more thread. I added 1" of thread using my die set, and removed 1" of thread from the end with a sawzall. Take a grinder to the rough edge so the nut goes back on freely.

Permanently attach the new bracket to the frame rail. It is wise to remove the tranny cover from the tub so you can weld the top of the bracket to the top of the frame rail. If the tub is off of the JEEP, drill holes to bolt the top of the bracket to the frame rail. Either way, you have to have the support there.

I mounted the plunger rod back to the brake pedal arm before I mounted the new M/C to the bracket. It seemed to be easier to do it before the M/C was mounted. Place the rubber boot you saved from the old M/C on the new M/C and bolt to the bracket. Make sure the plunger rod seats properly before tightening the bolts.

You are now ready to add your brake lines and bleed. Please note, my JEEP has 3/16" brake lines and the new M/C is equipped for 5/16". You will need adapters to make these fittings match up. Be sure you have the clearance with the bellhousing before you buy fittings for the M/C. I had to buy 5/16" brake line and reduce it at the flex lines on the axles.

Dennis Davie