If you have cruise control there will be a valve where the brake pedal's push rod goes into the power booster(in the cab).
It will have a vacuum hose attached, that hose goes to the cruise control's cable actuator in the engine bay, it uses vacuum to control speed, when you press on the brake pedal vacuum is lost and actuator is disabled, it is a backup safety measure to prevent run away cruise control.
This valve can leak and make noise.
Then there is the Atmosphere valve in the power booster, it is also at the end of the brake pedal's push rod, when you press the brake pedal the push rod pushes open Atmosphere valve, letting in the 15psi air pressure at sea level, the engine vacuum pressure, -18 to -22psi, in the power booster "assists" you in pressing down the pedal.
Atmosphere valve could be leaking but as above post suggests a noise would indicate the diaphragm in the power booster would have to be leaking and would lose "assist" pretty fast after engine was shut off.
Just to be clear the loss of "assist" would be AFTER engine is shut off.
Power booster has a Check Valve on the engine hose, if diaphragm in the booster is "air tight" then AFTER engine is shut down you should be able to "feel" the assist on the brake pedal for 2 or 3 pushes on the pedal then pedal will become increasingly hard to push down(vacuum pressure is gone).
If you shut off the engine and press down the brake pedal once and then it gets hard to push on the 2nd time, your diaphragm and Atmosphere valve are leaking.
And just as a "heads up", 4.0l OHV engine does not react well to overheating, if you ever see temp gauge getting up to 3/4 pull over and shut down engine quickly.
The heads WILL crack when over heated, no do overs it WILL happen the first time and every time, they have a thin casting between valve seats, but they will run forever, UNLESS you over heat them.
So a $50 water pump repair or bad thermostat can cost you over $1,000 if you try to "make it home" or otherwise ignore temp gauge.
Pull over let it cool off and figure out what is wrong, you can drive it again a short distance then shut it off again.
If it takes you 3 hours to get home, but you saved the heads then that's $333 dollars an hour you just made/saved
Engine running slightly warmer can also mean a Lean fuel mix, but you should get a CEL(check engine light) if that was happening.
Does your CEL work?
Does it come on with the key and then go out after a few seconds?