When moving slowly forwards or backwards the brakes won’t engage when pressure is applied. When backing up and moving forward in a parking lot almost hit a car. When driving on the highway at a normal rate of speed, the car slows down just fine is when I come to a stop and I move forward slowly the pedal becomes real stiff and keeps falling for 2 to 3 feet before I can get it to stop, I just replaced the master cylinder, and the problem continued

 

I would check for a vacuum leak of some sort. The brake booster works off engine vacuum, and the vacuum increases at higher engine RPM. When you replaced the master cylinder, you mounted it to the brake booster. Check the vacuum hose from the engine to the brake booster, and there is also a one way valve at the end of the hose where it pushes into the brake booster that can fail. I had to replace the booster on a 2003 Yukon that failed due to an internal vacuum leak, before we got rid of it a few years back.