Why does the parking brake light keep coming on my 96 chevy k1500?

You left the door open for a lot of things to cause the red warning light to be on. First check if the parking brake pedal is fully returned against its stop by pulling up on it by hand. If it drops a little on its own suspect the main parking brake cable is stretched and needs to be adjusted. When you replaced the master cylinder, did you bench-bleed it before you installed it? I have a method that eliminates the need to bleed at the wheels, but if you did that, there's a chance there is air in a line yet. That can cause uneven pressure buildup in the two hydraulic circuits which will trip the pressure-differential valve and switch and turn on the light. The last thing is if your truck has a low-fluid level warning switch in the master cylinder reservoir, be sure the level isn't low. The only time the reservoir should be filled is right after you install new front brake pads. That's when the pistons are pushed all the way back into the calipers which pushes fluid back into the reservoir. From that point on it is normal for the fluid level to go down as the pads wear.

The noise in the rear needs to be addressed. It could be as simple as you didn't have the drums machined and the new linings are grinding on a ridge of rust. That will go away on its own shortly. If you didn't use high-temperature brake grease on the six "lands" the shoes on each side ride on, they can cause a squeak, typically when you release the brake pedal. That is simply an annoyance, not a safety issue. If you got any type of grease, including fingerprint grease, on the linings and friction surfaces of the drums, that grease will soak into the porous linings and drums and cause a squeal that will never clear up. Any hint of grease must be washed off before the linings or drums heat up.