Having an issue with brakes not working properly. When truck is running, first push on the brake pedal goes all the way to the floor and does not provide much braking power. If I release the pedal and allow it to come up, and then press it down again, it will firm up with each pump and brakes begin to work effectively.

If the truck is off and I push the pedal, it is much tighter. Then when I start the truck (and leave the pressure the same), the pedal will go down to the floor.

I have not done any recent repair work on the brake system. This occured all-of-the-sudden as we were driving down a mountain!!

This seems like the master cylinder, but want to see if there's any way to know for sure before I replace the cylinder, bleed system, etc.

Any help would be much appreciated. Thanks!!!

Bobby