A customer of mine called and said his brakes locked up on his Ferrari 365GTC/4. This car was restored and he’s had it for about a year but this past week on a drive he started having brake issues. They locked up so badly, he had to have the car towed back to his house where it was waiting until I could take it to figure out what happened.

When I got the car, the brakes had released, and after I rolled it into my shop, I tried to get the brakes to lock up without any luck. I tried pumping the brakes. I tried running the car and pumping the brakes. I tried everything, and eventually ventured out on a short drive to see if the brakes would lock up. I drove the car on longer and longer circles away from my shop, but the brakes just wouldn’t lock up!

I did notice however, that the booster wasn’t actually working. Even though the brakes would stop the car, there wasn’t much assist happening from the booster. Disconnecting the vacuum hose from the booster made very little difference, so I knew at the very least, I would probably have to rebuild the booster.

If the calipers were the problem, it would have locked up again pretty quickly, and a bad rubber flex hose would have done the same thing. A bad master cylinder usually sinks to the floor when it fails, and if there was an internal valving issue, it would have shown up long ago, not a year later, so I suspected something was going on between the booster and master cylinder.

Once I got the master cylinder separated from the booster, I think I found the problem. The rubber seal between the two units was not the right size. The one on the left is the correct size for this booster, and the one on the right was on the car.

Although the inner diameter is close to the right size, the outer diameter was larger, not to mention the shape was different than what is correct for this booster. I also found some petroleum based grease coating the seal and may have swelled up the seal to where it popped out the steel disc that sits inside the seal. Either the seal was completely wrong, or the grease caused the epdm rubber to swell.

You can see how much larger the seal is to the correct one. I think what happened was when the steel disc that sits inside this seal popped out, it sat crooked to the master cylinder pushrod and didn’t allow it to retract all the way, causing hydraulic pressure to the build in the system and not release. By the time I got the car, the seal had completely fallen out.

After testing the unit on my brake booster tester with the correct seal, it still wasn’t working properly, so I disassembled the booster to see what else was wrong. The main power valve was showing some wear and from the mark on the rubber seal so air might have been leaking past this seal.

I replaced all the seals, and completely rebuilt the booster.

I put everything back, bled the brakes, and went driving. The brakes are much more responsive and still no signs of locking up. From what I found inside, I was pretty confident I found the issues. I drove the car for a couple of hours and everything worked fine. It was nice day, it was a nice car, and it was now ready to send it home!