If the cylinder was empty, then there is likely to be a leak somewhere. Other than leakage, the only other reason for fluid to go down is if the shoes wear or go way out of adjustment, but there would have to be significant shoe loss to empty a master cylinder.
Most likely you have air in the lines. If you can even get one front and one back to open, then you should be able to achieve reasonable braking. However, if the vehicle is going to be subject to a safety inspection prior to licensing, then you are going to have to do it right.
You may find that as the vehicle sits for a couple of days, some of the air will make it's way out of the lines. But that still will not fix what needs fixin.
At a minimum, a new master cylinder and 4 wheel cylinders will only set you back a couple hundred. If your jeep is a Cdn2, the parts are very common. If it's a Cdn3, then the wheel cylinders are a little harder to come by.
At the same time you are changing the cylinders, also replace any of the flexible lines if they are showing any cracking. And as well, while the drums are off, it's usually a good time to freshen up the grease in the wheel bearings, as well as clean or replace the shoes and drums.
But that last paragraph starts to get a bit more expensive, and requires a bit more mechanical knowledge.
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