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Discussion Starter · #1 ·
2006 Volvo S40 T5.

Don't seem to have an issue with the vacuum pump coming on and staying on (immediately after shutoff), but do have a constant hissing that is audible with no brakes applied (it's definitely coming from under the dash in the area of where the brakes are applied), that gets quieter and seems to stop with brake application. I've read through every thread on this topic, and haven't found a surefire test to tell if its the brake vacuum switch verse the brake booster - anyone have any tips?

Given that the brake booster is a much more difficult job, and a $500 part (assuming I can find it), hoping for a test to determine it's the switch, which also seems to be a common problem.

Thanks in advance for the assistance.
 

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Interesting find. I also recently noticed a faint hissing noise under the dash that stops with the slightest touch of the brake pedal and when bringing the vehicle to a stop. It’s so faint I might have considered ignoring it had it not been for your discovery.

A little research on the topic indicates that this noise is characteristic of a worn plunger seal at the back of the brake booster, which is why you hear it in the passenger compartment. The only solution is to replace the brake booster as Volvo doesn’t offer a replacement seal.
 

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Discussion Starter · #3 ·
I'm going with the Booster and the brake master cylinder . . . I had some fluid disappear awhile ago, I'm wondering if it is sitting in the booster.

Not sure how long this has been going on, but now that it's possible to drive around without the air conditioning on it is obviously audible (without the radio). I'm guessing it was covered up for awhile by the fact that the AC was on throughout the summer.

Also, LMGVolvo pointed out to me that when replacing the booster there is a pin that is likely destroyed. Volvo part number 31302606. I bought my master and booster at FCP Euro (even though the booster was cheaper at some other places), but couldn't find the pin there - got that at Tasca.
 
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