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Q: Brake Booster potential leak.

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After noticing a slight to moderate smell, which sort of smells like melting crayons, I dug around inside the cabin of the V70 to find the source. I discovered that there is a very slight leak around the brake pedal and push rod assembly where the brake booster passes through the firewall. Seems that hydraulic brake fluid is seeping through the booster, maybe around it, such that the grey bellows around the booster and push rod are wetted. Slight drops have traveled from there to the steering column cover. While the fluid is not particularly smelly when fresh, my assumption is that the fluid is breaking down causing the odor. Brake reservoir is still filled to appropriate level. Clearly something needs to be replaced, questions are... Can the brake fluid travel past the booster into the cabin? If so, does it travel through the booster or around it? Can the booster be replaced without removing the MC, it seems not? Is there a way to check the seal of the MC without removing it?

My car has 108000 miles.
My car has an automatic transmission.

A: Hi and thanks for contacting YourMechanic. ...

Hi and thanks for contacting YourMechanic. If the brake pedal is easy to use when the engine is running and is stiff when the engine is off, then the brake booster is working fine. The oil is coming from the master cylinder rear seal and the master cylinder needs to be replaced.

Make sure that when you replace the master cylinder to bench bleed the master cylinder to remove all of the air from the inside of the master cylinder. Then after you hook up the lines, you will need to bleed the brake system to ensure that there is no air in the system. If you need further assistance diagnosing your brake system, then seek out a professional, such as one from YourMechanic, to help you pinpoint the problem and follow through with repairs.

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