before you do anything, check you hydraulic mineral oil. its the #1 problem on these vehicles, Fill it up if empty and go from there.
Its becaue the slave cylinder is has a leak, and pressure is lost when you press on the brakes.
under the dash about 6inches above the brake pedal under the dash about 6 inches above the brake pedal
will be attached to the brake pedal where the master cylinder rod is
It is behind (and touches) brake pedal.
brake leakage? or vacuum assisted vacuum leak?
Under the dash, on the brake pedal arm mounting bracket.
Look under the dash on the driver's side, half way down the brake pedal. When you depress the brake pedal, the pedal moves away from this switch, which allows the brake light pedal circuit to complete, and your brake lights come on. This brake light switch is always hot/on, as required by federal law, so that even with no key in the ignition, if you hit your brake pedal, the brake lights will come on.
The brake light switch is connected to the brake pedal and the brake linkage under the dash with a single bolt.
It is located on the brake pedal arm under the dash.
Probably bad or mis-adjusted stop light switch Should be located on top area of brake pedal
The turn signal flasher on a 1988 Cadillac is located under the dash on the driver's side of the vehicle. It is just above the brake pedal.
In park position Apply a soft pression on the brake pedal, Start the engine, if you feel a depression on the pedal, the brake booster is Ok. Otherwise be sure the vaccum line is OK. Good luck