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Puzzler about the man, the wife, and the empty can of brake fluid

The puzzler is about a car that idles rough at times. Turns out it was leaking brake fluid. The guys said something which surprised me. Maybe I misunderstood. But what I think I heard – as part of the explanation of the symptoms – is that it is possible for brake fluid to leak out of the master cylinder and into the power brake booster.

How is this possible? Isn’t the connection between the master cylinder and the power booster a simple mechanical one? In other words the driver pushes on the brake pedal, the vacuum in the power booster “gooses” the push rod input on the master cylinder with a little extra force is all. Or are there brake fluid containing tubes which connect these parts also? I just can’t imagine how brake fluid could get from the master cylinder to the power booster.

I think if the seal is bad around the rod that goes from the master cylinder to the vacuum booster, the booster can suck fluid from the master cylinder reservoir. Sixty years ago, Buick had this problem with models equipped with power brakes. The vacuum booster would suck the fluid out of the master cylinder reservoir and leave the unsuspecting driver without brakes. This was before the days of the recall.

Triedaq is correct.
Every '53 Buick equipped with the optional power brakes was a potential time bomb, just waiting for the badly-designed seals to fail and allow the brake fluid to be sucked out of the master cylinder.

Even though there were a number of deaths (and a lot of serious injuries), in those days auto makers could simply look the other way because there was no process in place for a gov’t mandated safety recall, and the mfrs essentially ignored problems that were potentially fatal.

But…yes, this can happen, although I have not heard of it taking place on any modern vehicles.

Sure it can happen, and it does fairly regularly. At the back of the master cylinder is the piston that pushes the fluid when you push on the brake pedal. When that seal starts to leak, the fluid can start to travel past the piston and leak out the back of the master cylinder.

Brake fluid eats away paint. The first tell-tale sign is paint peeling off of the master cylinder where the fluid is running down. That’s enough to recommend replacement. Recently I had a 2007 GMC with a mystery brake fluid loss–customer had to top off fluid 3 times in 2 months. Pulling the master cylinder showed a brake booster filled with brake fluid.

I understand that the MC can leak. I had a VW Rabbit year ago w/o power brakes and when its mc leaked, I’d find brake fluid accumulating under the brake pedal, under the carpet. It was leaking from the rear push-rod seal, like @asemaster says. So I understand that part, how fluid could leak out in that area. But how does it get into the booster? That’s what I don’t understand. It seems like any fluid that leaked from that seal would just drip down and fall onto the ground, rather than going into the booster? Is there a seal in the booster that has to fail at the same time for this to happen?

It takes two leaks, one in the master cylinder and one in the booster. Fluid leaks out, fluid gets sucked in.

I see. Thanks for the explanation everybody.

For many cars there only needs to be fluid leaking from the master cylinder. The booster is sealed to the master cylinder by means of an O-ring at the mounting flange of the master cylinder. In other words, the back of the master cylinder is in a vacuum all the time. Removing the master cylinder lets you look directly into the brake booster. Not all cars are designed this way, but many are.

Hondas were this way for many years. I would occasionally run into a DIYer who would bring his car in saying he replaced his master cylinder and now there is a loud hissing sound and the engine idles real high. He didn’t get the seal on the master cylinder on right.

I had a '71 Saab that did this. Kept adding brake fluid and none ever came out! I pulled the master cylinder and sucked a quart out of the brake booster.