Yep. Firm pedal feel when the engine is off is because you have power brakes. Unless you have a gaping hole in a line, you're going to have a decent pedal feel with the engine off.
If the pedal sinks to the floor with your foot basically just resting on the brake pedal with the engine running, and you're not seeing any leaks anywhere, the only possibility really is the master cylinder bypassing. You won't see any fluid loss because none is being lost. It's going from one side of the reservoir straight into the other. You've got a 2008 truck with over 100k miles. If you haven't flushed the brake system at least twice in that time, I basically guarantee it's the master cylinder. A chunk of sludge has worked its way along the master piston and between the seal and the wall, allowing fluid to go from one side to the other instead of down the lines.
When the pedal won't hold pressure, it means it's losing pressure. There are only two ways for that to happen: a leak or bypassing. Check the lines front and rear and underneath. Check the diverter and proportioning valves. Check the rear calipers as well.
If you have no leaks anywhere, and the master isn't bypassing, it will be the first time for me to hear about something like in more than 40 years of working on brakes.
Saturday, August 15th, 2015 AT 3:38 PM