After seeing the seal was out, I dug out my 68 Service manual and read everything about this system. You are supposed to fill it, while it is in place, or level. So that leaves quite a bit of room, for air and exspansion.
I am guessing that is why I blew the seal, due to overfilling it. It had worked flawlessly for 7 years, then I have a failure after replacing a part and adding fluid. Early on, I had broken a line (brittle 42 year old plastic)by jamming stuff in the trunk, but no other problems.