'12 G37 with Brake Booster Vacuum Leak

[Dentt42]

Vehicle: 2012 G37S, RWD 6-speed manual

I've been chasing this on and off since last week, and every time I think I've figured it out, it's a dead end. At idle, there is an audible constant hissing coming from my brake booster. Locating the point of the leak itself is driving me up a wall, and I'm hoping for some guidance.

So far:
-Capping the hose from the intake manifold to the booster stops the noise completely, so it's definitely the booster itself that's leaking.

-The hose grommet at the connection from manifold to booster is not leaking. I've replaced it and even tried sealing it with black silicone to make 100% certain, and no change.

- The booster sensor (connected by a second smaller hose to the booster) is not leaking. I can unhook the second hose at the sensor end, and capping the hose itself does not change the sound.

- The brakes lose power assist as soon as the engine is off, so the booster is not holding vacuum for any duration of time.

- The clutch master was replaced by a shop 10 days ago, and I noticed the problem very soon after. I usually do my own work, and circumstances forced going to a mechanic.

-The leak is bad enough to repeatedly trigger lean codes P0174 and P0171 for banks 1 and 2.


Do I have a bad booster? If so, how do I test it before removing it from the car? Going to the shop I chose meant calling in late to work to drop it off and then again to pick it up, and my boss complained.

Thanks for reading if you made it this far.

[SouthArk370Z]

The FSM (link in sig) should have test procedure and more info about possible problem causes.

[Dentt42]

Originally Posted by SouthArk370Z

The FSM (link in sig) should have test procedure and more info about possible problem causes.

Looks like I may have found it:

Originally Posted by FSM

Check the air tight condition when the master cylinder and the brake booster is installed.
1. With a handy vacuum pump, apply vacuum pressure of −66.7 kPa (−500 mmHg, −19.70 inHg) to the
brake booster.
2. If the air tight condition cannot be maintained, perform the following operation.
a. Check the no dirt and dust are present on the brake booster and brake master cylinder mating faces.
Clean it if necessary.
b. Check O-ring on the master cylinder. If anything is found, replace the O-ring.
c. Check the air tight condition again. If the condition still cannot be maintained, replace the brake booster

I'll report back. Thanks a ton!

[Rusty]

With the engine running. Spray engine starting fluid around the connections. If the rpms goes up. You found your leak.

[Dentt42]

Originally Posted by Rusty

With the engine running. Spray engine starting fluid around the connections. If the rpms goes up. You found your leak.

I’m familiar with the trick, but I was concerned that getting a solvent inside the booster diaphragm would cause further problems.

[Rusty]

Originally Posted by Dentt42

I’m familiar with the trick, but I was concerned that getting a solvent inside the booster diaphragm would cause further problems.

If you are just spraying around the connections. It won't hurt it. You're not spraying directly on the diaphragm.

[SouthArk370Z]

If you are worried about using starting fluid, try something that doesn't burn. Nitrogen, compressed "air" in a can, CO2, etc.

[Dentt42]

Whelp, I found it, thanks to the info in this thread. The master cylinder O-ring was indeed the culprit. Spraying brake clean around the seam did nothing, but I reached a point that it couldn't be anything else, so I ordered a new one and got the master off on lunch.

It was way worse when I took it off, but the o-ring had gotten caught as the master was slid into place. This most likely happened when the clutch master was replaced by the shop I had to take it to.

The pic shows the two places it was snagged still deformed enough to not lie flat.




Fired it up with the new one in place, and no hissing sound. All I have to do is bleed it with my new Motive Power pump, and done.