The fact that your brake light switch is being engaged means that the brake fluid is under pressure . All 78-83 928s trigger the brake lights off the master cylinder . You have been driving around with all four calipers dragging wearing out the rotors and overheating warping them and wearing out the brake pads . It has nothing to do with the hand brake that is separate mechanical system with independent shoes inside the rear rotor in a special drum . The obvious question is why are the brakes being pressurized all the time ? . The most likely occurrence would be a failed brake booster which thinks you are pressing lightly on the pedal all the time . You can very carefully test this by disconnecting the vacuum line from the booster ( use extreme caution ) You could test drive in a safe location and see if the brakes begin to self apply with out the vacuum boost connected . BE VERY CAREFUL without power assist it is very hard to stop . Wally points are an excellent (as usual) in-depth discussion of various tests but focus on the defective booster or lack of free play at the pedal .