Brake Vacuum Booster Valve?

[Silvestris66]

I was checking oil on my 2003 XC70 (244k), and I noticed that the top had popped off this valve I'd never paid attention to before. Turns out it's the Brake Booster Vacuum Switch. I was able to snap the top back on the valve, but the thing is, I hadn't noticed any problem with the brakes, nor did it seem to make a difference when I snapped it back together. No codes were registered, and there's no difference in how the car runs.

I've ordered a replacement from FCP Euro, but I'm just curious about what this valve is supposed to do and why having the top popped off didn't have any noticeable effect on anything.

Maybe replacing it with a new one will do something???

[Astro14]

I can?t picture what you?re talking about - but I suspect it worked OK with the cap missing

[Silvestris66]

It's Volvo part number 31400692, seen here:

https://www.fcpeuro.com/products/vol...volvo-31265825

It sits just forward and to the left of the air cleaner looking from the front.

The top cap with the electrical contacts had popped off and was hanging by the wires.

[Silvestris66]

OK, I finally figured out what this switch does. In a low vacuum situation, it activates an electric vacuum pump to supply vacuum to the brake booster. After researching how it works and what can go wrong, I checked the vacuum pump 15A fuse and found it blown, so the pump definitely hasn't been working. For how long, I don't know. I can't find my automotive fuses tonight, so I couldn't try replacing the fuse. Maybe the pump is dead.

But here's the thing: I haven't noticed it at all. Maybe it's because whenever I start the car cold, I always wait for the idle to slow down before putting it in gear. I haven't noticed any change in the brakes.

So is this vacuum pump that important??