Spongy Sinking Brakes

So, I replaced my entire brake system due to too many trips to Baja. Rear brakes: replaced shoes, all hardware and drum. Front brakes: rotors, pads and calipers. After doing so, I flushed entire system and bled the lines to ensure no air in the system. (I actually flushed before I replaced all brake hardware). After bleeding all four tires I topped off the master cylinder and proceeded to take my Taco for a test drive. Before I turned car on I tested brakes and they were nice and firm and my e-brake was back to stiff as well. So I it up and went to pull out and my brake pedal became extremely spongy and even began sinking if I kept my foot on the pedal. As I was driving the truck would stop but not very fast and the pedal was nearing the floor.
So, my question is...does anyone know what the problem could be? I'm thinking my master cylinder is going bad?? My Taco has around 135k miles on it and its a 2003 4x4 V6.
Thanks for any advice or help.