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I have the stock single. It's leaking and I need to replace it. I had a bit of a hair raising time last night, when I was driving home, the front wheel cylinder developed a leak, and only the E-brake saved me. I now need to replace it as well. People like Herm tell me I should upgrade to a dual master cylinder. It's a bit of fabrication work (cut a bracket off the frame, shorten the rear pushrod, and drill the top so it can take the vaper line threaded insert). He says it will then give me a dual system, so if the front fails, I still have brakes in the rear. My mechanic friend said it's a waste, my brakes have a single line out, and if one cylinder fails, the whole system loses pressure.

For a daily driver the dual master cylinder conversion is the best way to go. Herm sells a good kit

I know, I talked to Herm. If I go with a kit I'll go with his. I just want to know, before I shell out the money, if it's worth it. Does it really split the front and rear brake lines into dual sections? My home mechanic says "your brakes don't work like that". Then again, I think Herm would know what he's talking about.

Unless it was explained wrong, get a new mechanic. Tell him to look at any modern vehicle. They have 2 circuits, front and rear proportioned accordingly.

Alright, I'm convinced, I'll get the dual conversion from Herm. My only question now, is what to do with the vapor line off the top of the M38A1 master cylinder. Any suggestions?

Reviewing all the mods you have made thus far it is plain that a stock M38A1 is not what you want. So just discard the vent tube and plug the fitting on the air cleaner for it.

Dan said:

I just want to know, before I shell out the money, if it's worth it.

In my comment in my earlier reply that is exactly what I told you!

For a daily driver the dual master cylinder conversion is the best way to go.

Dan said:

Does it really split the front and rear brake lines into dual sections?

No it doesn't split the front and rear lines into two sections. It isolates the front brakes from the rear brakes.

A dual master cylinder has two reservoirs. One for the front brakes and one for the rear brakes.

A dual master cylinder has a dual piston. One for the front brakes and one for the rear brakes.

A dual master cylinder has two separate lines coming out of it. One for the front brakes and one for the rear brakes.

I am trying to keep it as stock as possible, while doing minor upgrades that are both reversible, and yet improve it's safety as a daily driver.
I originally had very little idea what owning a military jeep entailed, but thanks to the advice of people like those on this website, I have learned it is a far different animal than the CJ's I've owned, and am endeavoring to keep it that way.

Oh, and what is the purpose of the vapor line on the top of the master cylinder?

It's a vent line. The civvy master cylinder lid has a small baffled hole to vent the master cylinder. Since the M38A1 was meant to ba able to ford deep water the vent and fording systems were placed there to help keep water out of the systems. This vent and fording system will pressurized the engine crankcase, tranny, transfer and bellhousing to about 1.5 PSI differential. The brake master cylinder is vented to the air cleaner as is the carb bowl and since the air cleaner is protected by the deep water fording snorkel the water is kept out of those systems.

All this is illustrated and explained in great detail in your copy of the TM 9-8015-1 which you should be using when tracking down this particular tube.

Getting back to your original topic then are you going to do as you said and keep it stock with a single chamber master cylinder or modify it for safety to the dual master cylinder?

I will go to a dual master cylinder set up. It's a minor and reversible upgrade. Is there something I can do to to vent the master cylinder into the air line to keep it's fording capacity?

Dan said:

Is there something I can do to to vent the master cylinder into the air line to keep it's fording capacity?

Depends on what style lid you get with the new dual master cylinder.





Here are just a couple of lid types. If the lid's built in vent can be drilled to accept an 1/8P nipple then you are all set. Some have very odd stampings with included vent forms that are not conducive to drilling and threading. These will usually need to have the built in vents plugged and a new threaded vent installed. Also some lids seal the two reservoirs separately which means you'll need two vent fittings and lines that "T" into one that then runs to the air cleaner.