Chevrolet antilock brakes

The brakes on my 1999 chevy suburban have a hard pedal but as I continue pressing it goes slowly to the floor. After the dealer worked on it the problem was worse. The front rotors and calipers were replaced and the rear drums were inspected. There are no leaks, I am not loosing any brake fluid. Bleeding the brakes does not show any air in the lines. The truck stops quite well from high speed. In traffic when I use the brakes to slow down with constant pedal pressure over a longer time period the pedal goes to the floor and the red dash light comes on. If I pump the brakes I can stop and the brake light goes off. When the master cylinder was replaced it made no difference. Could I have gotten a bad replacement master cylinder? What is the red brake light senceing?

It sounds to me like you may indeed have gotten a bad master cylinder, and that it's bleeding pressure back into the reservoir. Not 100% sure on a '99 suburban, but in my experience the red brake light senses differential pressure between the two independent brake circuits, front & rear. It's done in different ways, but one is using a spool valve that shifts when one side has low pressure, and must be "reset" to a centered position once the problem (usually a leak) is fixed.

GM has an $800 valve body in the system that is mounted down under the mid section of the vehicle----------has lots of brake lines on it and you can find it by simply following the brake lines---not real hard to get to but part is $$$$$$$$$$$$$$

Ditto on Jerry's post. But, it takes a tech II scanner to bleed that system. You might try making some panic (antilock activation stops) wet pavement works best. Do that a few times and then try bleeding the system as sometimes it will work.