just get a hobbes switch, they come in two forms of pressure sensing.
low pressure/vacuum. or high pressure/boost.
they are available at most parts stores. (they are like a low oil pressure switch).
i used one with a water injection system on a high compression gas engine..
'83' 2.2 diesel Ranger
'56' F100 project
(crown vic front suspension, mark 8 - 4.6, 32 vlv - v8.
rear end 'to be determined' .. mark 8 or cobra..)...