[Richard L]

The HFS-3 does not switch to low boost when the tank is empty but it will reduce boost if the water flow is outside the failsafe window.

This is because the tank doesn't become empty instantly. Instead, it goes into an intermittent state when it is approaching empty. If this is tidied into the boost drop, the boost pressure will react badly. So under these conditions, the water level led on the gauge will flash, warning the user the tank is approaching low level.

If the tank level is low before the the ignition key is next recycled, the HFS-3 will not switch on after 5-10 second. The yellow dash gauge led will state on.