The 'valve' is a rubber check valve with a long spring, in the bottom of the pump. In my book, I outline how to take it apart and check or fix it. Most of the time, since the rubber is a little hard from the years, it must be nice and clean or it won't seal well. They often have tiny chunks of [something or other] in them that either makes the rubber face not seal very well, or makes the plug stick in the slightly-open position, causing it to drain the tank in a few hours.
Often (even when the bikes were pretty new), use of high-detergent, 10w40 oils makes this situation much worse. Use 20w50 oils, or 20w40 in winter (15w40 or 15w50 if you ride a lot in cold weather) so the body of the oil helps to seal this port.
You can remove the pump by dropping the bottom pan, where you will find 3 bolts holding it up in the engine. Replace the 3 O-rings on the locator dowels when you do this: they are often hard now, and also leak some. There is another O-ring just like those in the end of the stopper valve's channel: they are all 15x2.5 mm size. Take off the end cap, remove the spring and plug, clean well inside and the parts. Reassemble with new O-rings, see if that helps: it often does.
The rubber plug (valve) is NLA from Honda, and I haven't made the aftermarket versions yet. But I will...
