I see what might be a regulator of some sort to support Drifter's idea. Does it connect to the brakes in anyway, for it to be a brake booster I would think that it has to attach to the master. Either way, I suspect you rebuild/repair by removing that center band clamp, separating the halves and there is probably a rubber diaphram in there that has gone bad.
Careful your character's showing
Yes, that is a brake booster. it is called a Hydrovac. It was a common vac booster for 50's and 60's medium duty trucks. As of a few years ago you could still get them rebuilt and new. I used to put them on my 1 ton and up trucks.
think of it as a vac booster that goes inline after the master cylinder. generally they were frame mounted under the cab. the fitting you have highlighted is for the vacuum source. those boosters usually had multiple places to choose from to run the vacuum line into it
That is an old in line booster. It boosts the fluid itself. I have seen a couple on early IH Travelalls. The biggest problem I have with them (besides being entirely obsolete) is they only work on single master cylinders. My first upgrade to any CJ is a switch to a dual unit, which is MUCH safer.
They work pretty much the same as the more modern version. Jeep daddy is right they are used on the single line MC systems. Fluid pressure from the master cylinder is sensed by the control valve (the small diaphragm). and boosted accordingly to the output side of the cylinder (the end with 2 fittings) one to the rear and one to the front. When the first dual chamber masters came out they used dual versions of these very briefly. They were used on mostly 1 ton to 5 tons, bigger than that it was air brakes. Imagine trying to stop a 1959 F600 with 15X4 inch rear brakes and 15X2 inch fronts with out power assist. This is what they had and it worked great. My F600 stopped on a dime.
I have never seen one in a jeep before. Personally I don't think jeeps need it.
You say its not hooked up. But I see brake lines running into the unit. if your hydraulics are still tied to that booster and its not hooked up to a vac source the brakes will be very hard. The fluid has to manually pass through all the valving. Get the brakes off the booster entirely and it should stop better. I suspect the booster is leaking vacuum that is why its disconnected.
That still won't get you power brakes (to do that you'd have to convert to hanging pedals and mount a firewall mounted brake booster). But it will modernize your braking system and improve it enormously.
74 CJ5 complete frame off restoration