i've had my tires for about a month now and one of my tires is losing pressure when the cap isn't on. i can notice a hissing sound whenever i take off the tire stem cap. does anyone know how to fix this without taking it to the shop? thanks

Why are you risking your life for a 20$ valve replacement? At best all you need to fo is tighten the valve in the stem. You need a special tool to do this but I have seen some metal valve caps that have this tool.

anyone know how to tighten the valve in the stem?

err valve stem tool? avail at your local parts store.