I must respectfully disagree with joek3167. There is no pressure relief on a closed system underbody. The seal is actually very easy to come by at any industrial seal supply house. You'll find one in any large metro area. They will only need the ID/OD and depth of the seal area so supply something that will work, probably a chevron style. They are pretty cheap also.
I used to always drill a 3/16 hole thru the pipe plug you mentioned and put a cotter pin in it to keep the bigger contaminants out (butterflies, hummingbirds, etc) as this would allow for a degree of expansion of the oil and allow any air to escape w/o making big issue out of service to bleed the system. It always worked for me but I am getting kind of old and memory is only a fleeting memory...
