Check the master cylinder and for external leak at wheel cylinders and calipers-start here
December, 16, 2012 AT 2:51 AM
Check for a brake fluid leak first. If none are found; try bleeding the master cylinder.
Have an assistant to slowly push the brake pedal down, while you loosen one of the master cylinder fluid lines until fluid streams out. Mak sure that the assistant does not let the brake pedal up until you have close the line.
Repeat this until a steady stream of fluid comes from the line. Repeat on the other line as well. Be sure to keep the master cylinder reservoir full of fluid throughout the process.
After bleeding the master cylinder, bleed the brakes at each wheel.