Kris, (Laurium, MI)
Kris, it's quite likely there is an air leak somewhere in the hand pump itself or in the hose between it and the tank. Check the hose carefully for any splits or holes. Some simple hand pumps were not usually constructed under the strictest of tolerances and it may simply be faulty. Though replacement hand pumps are readily available at any RV accessory store, it may be time to upgrade to an automatic 12-volt demand pump, even for a single sink configuration in a truck camper. It would depend on how much time you actually spend in the RV and if you have the space to install it.