Been trying to get a more firm and better brake pedal on my 2000 Silverado Z71 for awhile now, Replaced all Calipers, Pads, and Master cylinder.

It's like the first 70% is useless at any speeds over 40mph. Takes about 80%+ to get a decent nice stop. At a stop light the brake has to be pressed 70%+ in order to keep it from moving. I tried bleeding it my self the old method way, RR, RL, FR, FL... Running a clear tubing hose the same way to the master cylinder, finally gave up and took it to JustBrakes to get the system bleed and it's still the same.
Could it possibly be that I got a bad "New" Master cylinder? (There's no leaks in the brake system)

 

time for hydroboost.

 

You should take it to the dealer and let them bleed the ABS module. It requires a Tech II do do it. Others have had success by going on a dirt road and slamming on the brakes repeatedly to lock up the wheels forcing the ABS into action, freeing up air bubbles. I've tried this without success, but others say it works.

If you replace the master cylinder there's a very good chance you got air into the ABS module.

Hydroboost would help but doesn't remove the air from the system.

 

Really? because I thought hydroboost worked better with air in the lines.

 

sounds like your vacuum booster is dead. my 86 k10 did the same thing and it got to the point where I had to put the truck in neutral to keep it from rolling forward at a light. wore out my leg trying to keep it from moving and trying to stop it.

 

Did you bench bleed the new master cylinder before installing it? This typically needs to be done to get all the air out of the master.