Could be a vacuum leak
Could be a defective power brake booster
Could be a bad check valve on booster
One sign of a brake rotor going bad on a Jeep Grand Cherokee is a vibrating steering wheel. Another sign is that you feel your brake pedal pushing back.
A short in the brake light circuit.
There should be a contact switch just in front of the brake pedal up under the dash panel.
either behind or in front of the arm of your brake pedal. Follow the brake pedal up until you find a box, either in front of or behind the brake pedal. You will find that the box has a button on it that is activated by your brake pedal, that is the neutral safety switch.
The brake pedal vibrates.
The brake light switch is behind the brake pedal.
Check the sensor on the brake pedal arm.
Defective or mal-adjusted stop light switch? Usually located at the upper end of the brake pedal under dash area
if you look under the dash at your brake pedal if automatic trans,and if its standard look at the clutch pedal and look for the switch,this is not a hard job to do but it will take patience.
out of brake fluid?
Just above the brake pedal. The switch sets against the arm of the pedal.
Adjust or replace the stoplight switch at the upper end of the brake pedal under dash area