Power Steering pump leak

I have been chasing down some leaks and problems with the power steering and assotiated hoses. During my replacement of the O-ring I noticed alot of PS fluid on top of the reservoir coming from under the cap. Sure enough after wiping it up there was a mostly continous seepage from under the reservoir cap when the engine is running and warm. Do I need a new cap, rubber gasket in the cap, or is something else wrong to be causing the fluid to push its way out of the cap. I am not a power steering expert but I dont see what could be causing that pressure. Thanks for any help.