If you can pump it without any leaks, but it doesn't seem to hold pressure, than it must be a malfunctioning pressure relief valve. It is located at the base of the pump, where two white plastic tubes join together before connecting to the reservoir. One of those tubes supplies water to the pump, the other tube is the pressure relief valve, which jettisons water back to the reservoir. It operates with a simple spring which has probably rusted through in your case, leaving the valve permanently open. It is possible that the spring is still intact, and that the valve is simply wedged open. You can easily test this by opening up the case, removing the reservoir, and shaking the pump assembling/lightly knocking on the outside to see if you can loosen/reset the valve. Of course, complete removal of the valve is another option, but be warned that over-pressurization of your gun is a possibility in this case.
To get the gun open, you will need to remove the nozzle cap, which is glued on in the case of some CPS 1000s. Being 16 years old at this point, the plastic is likely very brittle, and may easily break, so be gentle in your efforts to remove the cap and open the gun.
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