Power Steering Leak at Reservoir Cap

[Frosten]

I have 4.3LX mercruiser from 1992.  Everytime I go out I have power steering fluid in the bilge.  I determined the issue is coming from the reservior cap.  I noticed there is not an o-ring on this cap and it just twist locks.  My question is, if I add an o-ring will this harm something? in other words, could the spraying of the fluid from under the cap be an indication of a more serious problem, or is it that after 30 some years the o-ring or gasket has been lost?

I tracked down this issue by zip tying paper towels around all the hose connection points on the reservior, pump, and cooler.  Only the towel on the cap was soaked through and through.

[Fishtank]

Hi Frosten.  I see you have an old post here but I am having same problem.   I was losing power steering into the bilge.  
Replaced pump, still leaking.  Found a crack in high pressure hose.  Changed all hoses.  Pressure tested fluid cooler, all good.   Put everything back together and I seem to be getting fluid leak from around the power steering pump cap.  This is a new cap that came with the new pump.

Anyone else ever have a similar issue?