SOURCE: 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.
Fig. Fig. 1: Vacuum booster mounting
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i have checked all switches and they are working
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