I just replaced the 5 o-rings in the top of my 300d pump for $15 including postage for the o-rings. The o-rings were hard to find w/o buying the entire $25 kit. I got them from Midwest Fuel Injection @ 1-708-532-1102.

When, as, and if you install them put some Vasilene around the o-ring and slide them in the top of the pump as square to the face as possible. Clean the hole first and lube it a bit. I nicked the first ring I installed and it leaked like mad.