Brake Problem 78 991SC
 

In the past the brakes on my 1978 911SC were excellent and responsive. When I got the car I had them service to include flushing the system and changing out the fluid (within last two years)

Driving it this week I got the scare of my life when I pushed on the brake and it went half way down before any resistance and barely braked, I pumped it and it worked. I drove it on some along a safe route and duplicated the problem.

I checked the brake fluid level and it is good (no loss of fluid level) I did note a little brake fluid on top of the Master Cylinder Reservoir. The cap was tight as it should be.

I suspect a brake booster going bad or a master cylinder, but will not drive it again until I can get it to Bob Hindson Racing, Inc. in KC.

I would NEVER trust someone else to bleed my brakes - Grandpa taught me when i was about six, and brakes/tires/suspension have always been safety priority #1.

Do it yourself, just to be sure. It's not hard - you probably just have an air bubble or something. You should flush and bleed every year or so anyway. Short of tires, brakes are the best safety gear you have, and some dude at a shop doesn't care if you crash/burn.

Wow, with all due respect , the above advice is TERRIBLE.

If you are not technically comfortable leave your brakes to a profesional as it is the single most important piece of your car.

Now that being said lets move on to your issue .. As you said you had your fluid changed within a couple of years so i doubt its related to anything they did. Porsche does recomend flushing once every two years under normal use so you are about due anyway .. Sounds like your master cylinder is toast , as the fluid is bleeding past the seals and going back into the reservoir.

I doubt its yourbooster as that would just make it hard to brake , dropping pedal is classic master cylinder.

Now since you have a 78 I would humbly suggest you inspect the date coding on your rubber connection lines from the hard line to the calipers. If they are over 10 years old this might be a good time to replace them if they werent done last time .. They could even be the originals.

If you feel confident you could do this yourself, but since you took it in last time i would say play it safe and take your car in . Most people who start off with brakes start off with pads, then move to bleeding, then caliper and master cylinder repair.

Good luck with whatever you decide ..

hahah... if you're not "technically comfortable", then driving a 35 year old car is probably a dangerous idea to begin with!

Do you notice any change in idle when you use the brakes, stationary? May help understand if it is the booster (idle change) or the MC (no change).

Interesting, and scary, that you were able to revisit this behavior in such a short time after the initial scare. This is not good - take it to someone you trust. Sounds like you have that at hand - feel free to use your AAA or similar to flatbed it over - that is why you are paying the $$ to them!

Don't sweat using other's experience to sort your ride. Ask questions, learn, and excel. Remember, they are the ones test-driving your 911 and will be the ones to pay should their fix fail.

Iceman nailed it. Your master cylinder has failed seals on the brake piston. What is a real pain is that the master cylinder is easy to repair with replacement seals but you cannot just buy the $1 rubber seal required. You need the whole kit or the whole master cylinder, $100 or $200 respectively. What happens is that when someone bleeds the brakes, he pushes the brake pedal to the floor to remove air from the lines. This action forces the decades old seals against cylinder surfaces that are coated with crud from years of sitting without being touched by the brake piston. The crud makes micro tears in the edge of the rubber seal on the brake piston and the fluid can then leak past the seal when the pedal is depressed. Bottom line is you need a new master cylinder unless you want to rebuild it yourself.

Thank you for the confirmation on the master cylinder.

I have done the complete brakes on my 83 Pininfarina (Fiat) to include the master cylinder, however I will let the professional porsche mechanic do this one as it was my father's car and I have spared nothing at each step. That and I don't have the hours it would take me to complete at this point in the summer.