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Last week, after a long-ish (50 miles on the highway) drive, as I shut off and got out of the car (2008 C70 with 105K miles) I heard a fairly loud, unfamiliar whirring noise coming from under the hood. Popped the hood and there, in front of the battery and in plain view, was a metal 'can' with vacuum hoses attached to it, buzzing away loudly. When I touched the flat found top I burnt my fingers, that's how hot it was. To get it to stop, I had to undo the wiring underneath.
After research, including here, I found this is the electric vacuum pump that builds vacuum when the engine is not running or otherwise not generating enough vacuum to boost the brakes enough. It's only supposed to come on sporadically and is activated by a pressure switch that is part of the vacuum hose circuit behind the battery box, right onto the brake booster. What apparently happens is that the switch fails, causing the pump to run all the time, until it basically self-destructs (which mine was clearly in the process of accomplishing).
So I looked for the replacement part and found it here. It's the hose with switch integrated. Separate switches without the hoses are offered, but since the whole assembly is heat-shrunk together, I don't think getting a separate switch will work for the C70:
https://www.ebay.com/itm/Genuine-Vo...&_trksid=p2057872.m2749.l2649&redirect=mobile

Yesterday I swapped out the part. You have to remove the battery + box to get access to the old part. It just pulls out from the brake booster (apply a little force as it's under vacuum) then 'snake' the old hose out from behind the brake master cylinder. When I got the new part in place and connected, and plugged in the vacuum pump, it ran for about seven seconds (to restore the vacuum I had removed when I took out the old part), then stopped. I went for a test drive and the pump stayed off, as it should.
So it seems this fixed the problem. My old pump seems to have survived its ordeal, so for $110 I should be good to go. I checked the old switch, which seemed to be operational, however I imagine it was sticking or otherwise on its way out.
 
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