Dave,
I have used the pressure switches and found the opposite to be true. Indeed, when I used one in my hydaulic clutch line as a safety interlock where the engine will not crank without pressure on the clutch pedal, I found that only lightly laying ones foot on the clutch pedal was adequate to engergize the starter solenoid. (But, if you have to put your foot on the pedal, shame on you if you don't disengage the clucth prior to cranking the engine, except in testing situations, of course, where a broom handle works quite nicely ;-) I suppose the source of the switch will make a difference in the pressure required to complete the circuit; I have used NAPA part no. SL134 throughout.
I have another of these switches on the clutch line near the transaxle. This switch or a microswitch, which senses when the transaxle is in neutral, provide a no-load signal to the engine management computer. The only downside I see to their use is the danger of leakage, which doesn't seem to occur too frequently. When they do fail, they almost never cause complete loss of pressure in the line. That said, a mechanically activated switch does completely preclude this possibility.
Lynn