Compression test:
Open up the gearbox, removing the spring guide and spring.
Cover the air nozzle with your right index finger.
Using your left hand, draw the piston back and with a good amount of force, slam the piston forward. If you have a good seal, you should actually be able to get the piston to come to a complete stop, held back only by pneumatic pressure. The more forceful your push, the more likely you should come to a complete stop. If you feel air releasing on to your right hand anywhere, you'll need to localize it.
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