Greetings,  Modern strain gauge sensors are available pretty
cheaply now, including amplified and compensated ones.  They produce
an output voltage proportional to the pressure applied.  Differential
sensors can be used for both pressure and vacuum or for differential
measurement as one would guess.

Proportional control can be based on this voltage [output from the
sensor] but it is not a trivial design.  Simple on-off control can
be achieved by driving a comparator circuit with the output of the
sensor, but hunting is likely to be a problem and other circuitry may
be necessary to control it.  Mechanical methods, as Robbie suggests,
are more within the capabilities of a person without advanced skills
in electronics.

Best regards,
Spencer Chase
Laytonville, Calif.
http://www.spencerserolls.com/