Norm
I just got back this late AM and now got to the board from a week long trip. I am trying to catch up now.
I would check all of the hoses for any cracks or leaks. I would also check the O-Ring again. I replaced one once and had it go bad within a day. It was my fault because of the way I had instarlled it. After that I didn't have any more problems. There are some other O-rings near the switch that I have had to replace and have let air into the system. If you need more info on that, let me know.
I would look for leaks in the system or where the fluid is leaving the system. If it is just the cap, it may be the pump. If the pump is bad, it may be making foam which is displacing the fluid and making it expand and leave via the cap. If you do replace the pump, don't throw it away. I would like to see if it can be bebuilt.