Sentence:  Some vehicles with air brakes also use a <WSD>wigwag</WSD> low pressure indicator. This is mounted above the windshield. If the air pressure drops below 60 psi, the signal arm will drop down across the windshield. You can't reset it (push it back out of view) until air pressure is brought above 60 psi. Don't operate the vehicle with the signal arm lever down. 
Meaning of the word marked with <WSD> tags:  A warning device inside the cabin of a truck that causes a mechanical arm to drop into view when the pressure in the airbrake system of the truck becomes too low for the brakes to be reliably deployed.