First off, nearly every 'Brake Problem' that rolled into the shop for years started with INSPECTING & adjusting up the rear brake shoes...
When the rear axle isn't doing it 20% or more, it seems like it takes for ever to get stopped.
Doesn't cost anything but a little time to pop the drums off, Inspect the Drums/Shoes for glazing and water damage, under spec parts, ect. which is so VERY common with trail Jeeps...
And to check install and adjust up the shoes against the drums properly...
Jeeps drive though mud & water, moisture causes glazing, and the inside of the drum and the pads on the shoes are supposed to be slightly ROUGH, not smooth!
Water also keeps the adjuster from doing it's job, so don't forget the 'Never-Seize' when you clean/inspect it!
Makes things MUCH easier to adjust when the adjuster isn't rusted together!
The SECOND step was to check the front pads and rotor.
Again, your pads and rotor are supposed to have a slightly rough surface, like 400 grit sand paper, if they are smooth, they are probably glazed and need sanding and/or turning.
Make sure the ramps the caliper slide on are lubricated, Again, get out the 'Never-Seize'. If you calipers are rusted or bound up on the caliper mounts, you will loose a great deal of your braking power.
(and you DO run though mud and water all the time!)
Pull the carpet or floor mat back in the interior so you can see where the brake rod passes through the fire wall, look for any signs of brake fluid on the INSIDE of your vehicle.
If you see ANYTHING that looks like brake fluid, the Master Cylinder has failed.
Unbolt the master cylinder from the booster and remove it a little ways (Don't need to break open the lines) and see if the master cylinder is leaking internally...
Check the small plastic valve at the master cylinder vacuum connection...
(Where the vacuum hose connects to the master cylinder)
If that one way valve or it's rubber grommet start leaking, the booster will be very hard to depress.
Check the vacuum hose for leaks.
.............
Advanced testing would be checking the vacuum line with a 'Bubble Trap' to see if the booster diaphragm is leaking,
Checking the line pressure at the Master Cylinder and at the wheels to make sure the master cylinder is putting out the required pressures,
And that it's getting past the Combination valve, which can occasionally go bad and block, or partly block, one end of the vehicle from getting all the pressure it's supposed to...
Using a vacuum gauge on your engine to make sure you are producing enough vacuum to make the booster work correctly!...