So I went to leave this morning and my brakes felt funny, like they weren't engaging until very late on the pedal so I put it in park, turned the car off and pumped the brake and notice that the pedal was not getting harder to push. checked brake fluid and it wasn't low at all, I don't see any fluid on the ground by and of the wheels (4 wheel disc). Could this be caused by moisture in the fluid? The only thing I can think of is that while I was rebuilding the engine over months I had accidentally left the fluid cap slightly ajar. It's got just over 1000 miles on it since the rebuild, if that is the issue do I need to replace anything else, or just flush all the fluid?

 

Might be a bad master cylinder.