Quick summary - I tested my theory of the dust monitor pump running backwards and spewing contaminate with a pump at EY and was able to get it to run backwards.

A day after the power outage Dave noticed that the PSL dust counts were still very elevated, so I went to check on the corner station dust monitor vacuum pump and found it in an odd state (alog86857). The pump was running very hot, read 0inHg on the dial, and had some loose connections. I turned it off, tightened things up, it turned it back on with good pressure. Thinking more about it after I had walked away, my theory is that the pump started to run backwards when the power went out and the low pressure of the vacuum pulled the pump in reverse. The power then came back on and the motor started and continued in that direction.

Today I wanted to check on the end station pumps, so I took this opportunity to bring a pump that needed to be rebuilt anyway with me to the end station and try to recreate my theory above. I found no pump at EX, so I went to EY where I found the pump completely locked up and the motor, not pump, very hot. I unplugged this, hooked up the one that needed a rebuild and plugged it in. It pulled to -12inHg. I then tried a few times unplugging the power, listening to hear if it started to spin backwards, then pulling it back in. Third time I got it and it was running while pushing a bit of air out of the bleed valve and the pressure read 0inHg.

I didn't test at the dust monitor end of this system if it was spewing out any contaminate, most likely the graphite from the vanes in the pump, but I'd guess if it was running backwards over night it would make enough for us to notice. I'm looking into check valves, in line filters, or some type of relay to avoid this in the future.