Where is my brake fluid going?

Question:

I have a 1979 Lincoln Town Car with a 400 cid engine. It has an all-original brake system and the front chamber in the master cylinder is losing fluid when it stands. I checked everywhere for leaks, including the wheel cylinders and the lines, and nothing is dripping anyplace. The car has hydro boost. When I fill the cylinder the brakes are fine but it loses fluid again. Do you have any idea where the fluid may be going?

Answer:

Unbolt the master cylinder and see if it is leaking fluid out the back. When a brake system is losing fluid consistently and there is no evidence of brake fluid coming from the wheel cylinders/calipers, hoses, lines, or connection points, the culprit is normally the master cylinder. When fluid leaks out the back of the master it may accumulate in the power booster or, for manual brake vehicles, on the inside of the car’s firewall behind the insulation and carpeting, where it can be difficult to see.