Go to a box store and buy a tee fitting air gauge and air, schrader type fill valve and adapt to a male or female thread at an out let. Hook up air pump and pump to 35 to 45 psi, don't go too high or you can hurt faucets or valves. Ever thing needs capped and sealed. After filling wait and watch the gauge see if it holds if not... start searching, valve packings love to leak, cheak with good leak detector soap, like for gas. Listen some times a good ear will guide you to a leak.