Just thinking out loud here, trying to detect when fluid is in a clear tube. A laser LED would work, but so might a narrow angle LED. Might be an easier way to do this.
Get the bias right to trigger a 5 second pulse from a 555 timer (for example) when the laser hits the CDS cell. Add a relay/power transistor, and give the bike odometer a click on the same pulse.
The vent is going to be a pain.
