The vent tube is required. If you go with a civilian cap (like the one that came with your master cylinder) it will have a small hole that achieves the same thing. If you were to simply remove the brass fitting on top of the existing cap, and put in a plug, then as the fluid heats up and expands from either the heat of the engine, or the heat of the braking, the pressure in the master will build up and cause the brakes to either drag or lock on.
Wile it is a good idea to check the master cylinder occasionally, you can also watch the most likely spots on the jeep for signs of leakage. That would be the back of the wheels where the master cylinders will leak, and the boot of the master cylinder where the fluid will leak if that cylinder is failing.
One thing that has become popular, although will be non-original to the jeep, are the remote reservoirs. This will be a plastic reservoir which you can mount on the firewall somewhere, and will include a bellows type diaphragm in the cap of it to prevent contact with the outside air. Brake fluid is hygroscopic: that is it will absorb moisture from the air. This way, your brake fluid will remain pure, and you can check and top up the fluid without having to be a contortionist. As well, there is less chance of you dropping dirt and debris into the master, which will cause premature failure.
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