Why is brake fluid puking out the resevoir cap??

[68basket]

I put this truck together and have been driving it for 4 months. Put about 3000 miles on it and today the brake pedal went to the floor. The front resevoir was empty so i filled it and reinstalled the cap. checked and no leaks at lines, hoses, or calipers. Pump the brakes and fluid leaks out around the resevoir cap. why does it spit out and how did it run it dry? this is the setup i have, should i ditch it and buy a whole cpp unit?

http://www.ebay.com/itm/1967-1972-ch...0c5284&vxp=mtr