What are the symptoms of the brake booster and master cylinder going out on an XJ? If I choose to replace them, are there any upgrades you would recommend?

 

booster failure usually shows by loss of assist (hard pedal) bad master bypassing will show through pedal going all the way to the floor.
get a cheap MC bleeder kit with clear tubing at the parts store and bleed the master on the vehicle, if you see bubbles, its probably bad.