Cast iron M/C? or the newer style with the plastic reservoir?

Plastic? Some have had problems with the plastic ones leaking because, IIRC, the caps wouldn't seal on M/C. Top plastic still had some seam flashing that left uneven surface that cap gasket couldn't handle.

Cast iron? Do a search here. Steel spring that holds cap on (or is yours bolted down?) may be weak or gasket shot. Gasket doesn't have any holes in it, does it? It should have a diaphragm that allows fluid level to go down some with brake pad wear.