Problem: Brake fluid loss with no external leaks visible - customer will comment on having to add fluid periodically but visual inspection of vehicle will yield no source of the fluid loss.
Cause: Brake fluid leaking into vacuum booster. This happens on vehicles that do not provide a vent between the mating of the master cylinder and vacuum booster. (See figure 70.1)
Figure 70.1A leaking secondary cup seal or a leaking pushrod output seal will allow fluid to enter the vacuum booster. Most master cylinder - vacuum booster assemblies are equipped with a vent in either the master cylinder (Figure 70.2) or the vacuum booster (Figure 70.3)
Figure 70.2 
Figure 70.3Solution: To diagnose this condition follow the steps below
1. Perform a visual inspection of the brake system looking for signs of external leakage. The leakage would have to be severe if the customer is adding fluid regularly.
2. If no external leaks can be found inspect the booster for brake fluid by removing the check-valve from the booster housing.
3. Use a coat hanger asa dip stick to probe to the bottom of the booster housing. check the end of the coat hanger for signs of fluid. There should be no fluid in the booster.
NOTE: It is a good idea to bend the end of the coat hanger around in a closed loop to prevent damaging the diaphragm in the booster.
4. If fluid is found, the source of leak will have to be determined. It is not uncommon to replace both the master cylinder and vacuum booster when this type of failure occurs. There is no practical method of cleaning the brake fluid out of the booster.
NOTE: it is possible for brake fluid to leak into the passenger compartment if the brake fluid level in the booster rises to a point where it can exit out of the air valve at the inlet end of the booster.
