I have been PM'd by a member requesting I describe the procedures by which the tap on a tap loader can be tested for leaks,so here goes!

Imagine I am holding an Airsporter,though the following applies to any vintage tap loader with an underlever.

Step one.
Fully open the tap and with finger off trigger cock the rifle and then return the underlever to its closed position.Now slowly close the tap and listen for air being sucked down through the barrel from the muzzle into the cylinder,filling the vacuum created by the rearward motion of the piston on being cocked against an open tap,which if it has a good airtight seal will have disallowed air from being drawn around it into the cylinder.If on doing this no inward rush of air is heard the tap is likely leaking.

Step two.
Cock the rifle again with finger off trigger and hold the cocking lever in the fully cocked position then fully open the tap.Now,very carefully and with a very firm hold on the underlever pull the trigger,you will potentially have the full weight of the spring to control as you slowly return the underlever to its closed position,all the time listening for the hissing of air being forced around the body of the open tap.On the best sealing taps that don't leak the piston will either stay stationary or move forwards very slowly the felt result being very little weight to control as you allow the underlever to return to the closed position.If on doing this immediately all the weight of the spring can be felt on the cocking lever the tap is a leaker!

Step three.
This time cock the gun close the underlever open the tap and load a pellet and close the tap ready for firing,but before you do place a ball of rolled up moist tissue paper firmly on to the closed off loading port.With this in place aim and fire the gun looking to see whether or not the ball of tissue paper flies upwards on a cushion of leaked air from the closed tap.If it does you have a leaky tap!

The above are the ways I try to judge how well a tap is sealing if I'm buying a vintage tap loader,but it does presuppose that the piston seal is doing a reasonable job in the cylinder and not allowing air back past it for the procedures to be of merit.

I do hope that all makes sense,by the way!