I would advise bringing your car to a mechanic as quickly as possible. Your brakes could fail. This puts you at a high risk for injury or even death.
I assume that you have rear drum brake. If so, the knocking sound which happens only when apply the brake is more likely cause by an out-of-round drum. Take your car back to the mechanic that installed them. This problem is not bound to go away.
Most likely it's the pads knocking because the rotors need to be machined or replaced. Alternatively, if you have rear drum brakes, an abnormality in the drums could cause the shoes to jump up and down vertically.
loose rear caliper or drum brakes, could be loose hardware or on disc brakes, possible warped rotor
Could be your brakes. If you have drum brakes not disk brakes that sound will happen when your shoes are worn down to the rivets. Some of the hardware such as springs, may have came off as well, which will make the loose metal sound.
If the steering wheel shakes, but to car doesn't, when applying brakes - your front rotors are warped (if you have disc brakes)...if you have drum brakes, the drum could be cracked or the pads worn unevenly If the whole car shakes, and the steering wheel only seems to react to that, when applying brakes - see above statement, but apply to the rear end. If the whole car shakes, including the steering wheel - all four corners are bad, and you're an accident looking for a place to happen soon. In any case, go to your nearest service center ASAP.
( 1 ) on each of the rear drum brakes
What about 1993 Pontiac drum brakes, please be specific.
This will greatly depend on what kind of brakes you have and which parts you are trying to replace. Do you have disc or drum brakes? If you have disc brakes, do you want to replace the calipers, pads, or rotors? If you have drum brakes, do you want to replace the drum, or the shoes?
Drum brakes.
No. Drum brakes expand inside a rotating drum to stop the vehicle, disc brakes squeeze in against a rotating disc.
The rear brakes on a 1984 Chevy truck are drum brakes.
Only the rear brakes are drum brakes. The front brakes are disc brakes. To answer your question, twist the end wing nut by your drum brake. It tightens and adjusts how much you have to press on your rear brake lever.