BRAKE BOOSTER?

A bad brake booster will result in a hard brake pedal, stiff and more effort required to stop the truck. Here are my questions to you, first, you said you replaced the master cylinder, did you bench bleed it before you installed it? If not, there IS still air in it, regardless of how much you bleed the brakes at the wheels. Second, when you bled the brakes, after changing the calipers and master cyl., did you also bleed the proportioning valve? Sometimes air can get trapped in the valve, and its almost impossible to get it out bleeding just the wheels. And lastly, when the brake light comes on, does the abs light also come on with it? Air can also get trapped in the abs control module too.

if the booster is going bad you wont have very much resistance in you brake pedal, but before you go to all of that trouble and spend alot more money try and do a really good brake bleed. you would probably be best off draining all of the ood brake fluid out and putting new in. make sure you start at the wheel ferthest away from the master cylinder first.