The ABS light is usually a speed sensor problem and is a GM nuisance . it should not affect normal braking. It might be a booster for power assist problem, which will affect normal braking (expensive). Get rid of that old GMC.
Check fluid level in the master cylinder, fill to the top if low with DOT 4 fluid. If you still have a spongy petal, look for any leaks in lines or in the calipers at all four wheels (you might have drum brakes in the rear in which case you will be checking the wheel cylinders for any fluid leaks. If no leaks are found bleed the brakes, starting from the furthest wheel from the master cylinder (passenger side rear)-open the bleeder bolt, and have someone push the petal to the floor and HOLD it there-watch the stream of fluid that comes out of the bleeder-if it is a steady stream, tighten it up and have the helper 'pump' the brake, if the fluid that comes out 'spurts' out, there is air in the line and that wheel needs to be bled again. Close the bleeder, have them pump it up, then open it again and tell them to push the petal to the floor again and HOLD it to the floor-close it, repeat as necessary until you get that steady stream. Once you have it, go to the next furthest wheel (driver side rear). Then do the passenger side front, then driver side front. DO NOT allow the master cylinder to run dry, you need to keep filling the resevior so that it is not empty as you do this, and have lots of rags available to mop up the brake fluid that will be everywhere. If you take your time, you can do this-it is just a messy job but not complicated.
Air in the brakes lines or out of adjustment,adjust brakes before bleeding.
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