air prssre switch?

i removed air pie from pressure switch an found a steady flow of air going to switch when hot water called for, i placed multimeter on air switch terms an got beep boiler running or not running i placed mtr on switch fitted to water pipe no beep running or not,when i blow down pipe to air switch it clicks an beep stops on air swtch but beeps on water switch if i blow down air switch pipe when boiler is running all stops