Q – My 1980 911 SC has an odd problem. I recently took it in for routine service and the brake fluid reservoir was almost empty. I have had the car in religiously for oil changes and I have the fluid flushed as part of my maintenance ritual. I never had any warning and the brakes seemed fine to me. So naturally I was concerned about the leak and I asked them to check it out. Then the shop called to say that there was no external leak in the brake system. If there is no obvious leak, where is the fluid going? Judy
A – One part of the system that can leak and leave no visible evidence is the power brake servo. So unless someone forgot to fill the brake fluid reservoir, and that seems unlikely, you need to have the master cylinder loosened from the servo and the servo inspected for a liquid. I am betting that the servo is sucking fluid right out of the master due to a failed seal. Even with the best of care, on a 33 year old car, sooner of later, rubber parts deteriorate. Another tip off would be to inspect the vacuum check valve for the booster. If the rubber shows signs of deterioration, bingo, you are hot on the trail.
Once the leak is found, a complete inspection of the brake system is in order. On low mileage cars, but older cars, the hydraulic system suffers from lack of use. After you begin driving it after a winter lay over, the brakes may seem just fine until you take a hot run out on the freeway. You may discover that, although they worked great when you left, that final exit ramp can be thrilling, not in the fun way, when you push on the brakes. You may experience a VERY hard pedal but very ineffective brakes. It has happened to me a number of times and it makes the hair on the back of my neck stand up.
I know I have mentioned these before, but for those of you who have older cars, it bears repeating. Old rubber brake hoses can appear to be in fine condition externally but they eventually swell internally and act as “check valves” for hydraulic pressure allowing pressure to pass through, but causing the fluid pressure to be retained past the line even when you release the brake pedal. This can cause that thrilling brake fade that I mentioned. If you have an older car, be aware of this phenomenon and test for it before you are surprised by it. Replacing rubber hoses that are more than 20 years old with Teflon lined stainless steel hoses is very cheap insurance.
Finally, later cars have a switch that is built into the cap for the brake fluid reservoir; your 1980 does not have that switch. Therefore, you do not have the benefit of an advance warning indicating that your fluid is getting low. It might be a good thing to consider adding a later model cap, switch, and warning light to the car! MC