Product details:

This compression tester has been completely tested at the factory for accuracy and leakage before shipment. If, at some point after usage, the tool does not hold pressure the probable cause is foreign matter in either the release valve or the valve core in the adapter. Usually by running an engine with the compression tester attached and holding down the side release valve, the foreign matter will be blown out. In the event that a valve core fails, replace only with a special valve core from the dealer.