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I've had the following intermitent problem since back in December and thought you would be interested.

Took a trip to upstate NY with my Beast outfitted with Dunlop SP Winter Sport 3D's. Unbelievable in the snow. Woke up to negative temps and had to be at a wedding not to far from the hotel. Fired her up and proceeded on our way only to discover we had no brakes. Super hard pedal, like no power assist. Couldn't stop, literally, so I took it slow and stayed in low gear. Thought I had a frozen brake line so I didn't drive again for the weekend and left the car in a heated garage. Leaving to go home all seemed fine.

Then after going to a carwash several weeks later, moderate temps, a similar situation occurred. Same hard pedal, no brakes but this cleared very quickly. Let my wife drive it a few days ago for an errand, no comments please, and she returned saying that my brakes weren't working well. I vowed to get it in for a diagnosis. Today driving to work after a 40 mile commute driving down the ramp into the parking garage again no brakes! You can roll pretty slow in first gear so that was it, needed to get this fixed now.

Today took it to my indy who thought over the phone it may be a brake booster problem. Told him it seemed to occur after the car was washed as I had washed it testerday. Had one of his techs pull the cabin filter off to expose the master cylinder and the booster and as he described it the cowl area that it sits in was full of water. This must be tied in with the windshield drain area. It was clogged so a lot of water was collecting there and the booster was partially submerged and due to it being in vacuum was sucking in water and either freezing or filling the booster. Both were drained and the drain line unplugged. All is fine for now. Booster or check valves may need further attention and I will report if this occurs again.

Thought this may be a useful alert to inspect drains in this area.
 

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If the water was actually getting into your brake system, I would definately flush the fluid. Can't hurt to do it anyway, but it will cause you problems if you don't and water is in there.
 

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That is kinda scary on many levels.

Drain the fluid resiviour.

Bleed ALL the brakes. Bleed the clutch.

Water boils at a much lower T° than Brake Fluid.

You don't want to learn about the compressibility of steam on a highway.
 

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If water was surrounding the bottom half of the booster, and the booster was sucking in the water, the brake fluid would not be affected. It is a sealed system after all.
 

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The first time I have problem with my brakes it will be going straight to the dealer for a checkup, that is something you don't want to mess around with. Nobody can say how bad your problem was because we don't know the car and were not there at the time but I would definately not leave it for a couple of weeks before i got it checked.
I am glad the problems happen to you in locations where heavy braking was not required.
 
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