I would start with the cap as it is only a $ 7 or 8 at the dealer. The gasket of the old one might leak. If that doesn't help, then people with more experience than I are needed to give advice in how to find the leak.
However, in my 87 there are check valves at the pumps that are supposed to let the fuel only go one way, that can be replaced and are not too expensive, There is also a fuel accumulator that keeps the pressure up which I believe is around $ 80. Check at the FastLane connection.