In some examples, a system includes a printhead including a printer fluid nozzle, an air valve in fluid communication with a channel of the nozzle to: (1) create back pressure in the channel when the air valve is closed and printer fluid is ejected through the nozzle and (2) release back pressure in the channel when the air valve is open, an ejector to eject printer fluid through the nozzle to form a meniscus of printer fluid in the channel when the air valve is closed, a pressure sensor to measure pressure within the channel, and an indicator in electrical communication with the pressure sensor. The indicator can, for example, indicate whether there is an air leak in the channel by determining whether the pressure in the channel is within a predetermined pressure range after a predetermined idle time following the formation of the meniscus in the nozzle.