[Whaler]

John 550
The only idea I have is that there may be some resistance in the contacts or that the actual triggering setpoint is getting higher with age/use.

I took a fresh new switch in to work and tested it out..it closed at 70 psi
The napa catalogue said it had an actuation setpoint of 60-120 psi....
Mine was at the low end obviously....could this change and require more brake force to close?? who knows?
They are cheap to replace and easy to test... with the switch out of the car, rig up a 0-100 or 0-200 # gauge and a pressure source and the switch all on a 1/8" pipe tee, use a meter on it as well.

Pressurize it slowly until you see it close on the meter ( resistance) see where it closes on the gauge
...if it is real high, replace it..12 bucks US or so....

Also reset is another consideration..where it goes back to normally closed state. after being triggered onset...you want no real differential between these two pressure other wise you will have a light that stays on after brakes have been let off.

Is your switch new or has it been in service long? Mechanical switches are only good for so many cycles in life. usually the high thousands

See the thread that I referred Dave to for part# napa

Tim