Air would not randomly enter the system. It would get in if the fluid ran low and air was pumped in or if the brakes were improperly bled. Also, if the brake fluid is really old, there could be mositure in the lines. If the fluid is old, then it needs to be bled anyway, you could try this first before the master cylinder is changed.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record