If the brake fluid is low at your master cylinder, it likely went someplace. Either the gasket on the lid is leaking or possibly your rear wheel cylinders.
If the brake fluid level in your master cylinder reservoir got low enough to suck air, chances are you'll need to bleed the brakes.
Pull the rear drums look for brake fluid soaked brake shoes and carefully partially open the rubber ends of the wheel cylinders and see if brake fluid runs out. If your careful to only partially open the rubber boots in the front (facing you) side of the wheel cylinder, they are a lot easier to get back on and seated.
I have a break problem also, there is a sign on my desk (given to me by my co-workers) that says coffee break, 8 AM to 5 PM, daily.
i have a little "brake" problem on my xj too. bleed the brakes, all is fine then ride for a week or so and then she pulls to the right and then her brakes are very low. bleed them again and then the same thing. it pull so bad sometimes that she almost pulls the steering out of my hand pulling to the right. the brakes then feel very uneven as if it is braking a little on a wheel at a time and then she feels unstable on the road. it is difficult to explain but feels like the wheel each get a turn so the vehicle tends to move all over the road. have bled the fluid right through till new fluid came out. still no diffs. could it be the master cylinder rubber or what? i got a set of VW POLO rear calipers to install on the back and hope this will solve the problem once installed. THOUGHTS??????
the fluid is full and i put new pads and on it.