I have a 2006 f150 and when I push the brake pedal it is extra soft most of the way down then it gets firm like it is supposed to and if I let it off the hit the brakes again it is still pumped up but go back to being soft after I drive a little. I looked and can't find any leaks and the reservoir is still where it should be. Could it be my master cylinder going bad or is there something else I could check first

 

Have you done any brake work lately that air could have gotten into the system? I would do a little more research, but i think you are right about the master.
Basically you have the master, the lines themselves, or the calipers to troubleshoot.

 

It should also be noted that over time (aprox two years), your brake fluid gets contaminated with moisture. Moisture does not compress as good as brake fluid, and may be the reason for your slightly spongy pedal. The entire system should be bled with new fluid. I do this on my vehicles every couple of years.

My calipers are in great shape; I've yet to have a failed caliper or seized piston in one.

Have your old brake fluid bled. I'm betting if you have no leaks, that spongy pedal will be gone.

 

I agree, I haven't had a bad caliper in a Ford yet, just wanted to throw that out there. I have replaced a lot of GM calipers on grand prixs, yukons, and 2500s.

 

Well I've had two calipers go bad on mine... So I wouldn't skip over that step. I would definitely check out the condition of your calipers, and flush the brake fluid. After that, maybe look at replacing the master cylinder.

 

Ok the calipers look good I'll try flushing the fluid first. How much fluid does it take to do a flush