Overheating Powder Special

I have a 1996 580 Powder Special. It has a radiator fluid leak at the water pump housing. I thought it was the hose, so I took it off and cut a little off and put it back on. In the process a lot of radiator fluid drained out. I filled the fluid reservoir with fluid and started it. It overheated in 2 minutes. I then read the manual and it said to lift the front and then the back and then keep filling it, etc. I did this and it ran for 10 minutes and started to overheat again. I pulled the thermostat and tested it in hot water and it opens and closes fine. I can't figure this out. There is still fluid leaking from the water pump housing. Can anyone shed some light on this, why it is overheating???

Would the seals leaking be causing it to overheat?

I did what the book suggested (lifting the front, squeezing and adding coolant, lift the back and fill with coolant, squeeze, etc.) and it ran for about 30 minutes before it overheated again. I do not have the "unpressurized" cap that allows you to run it without pressure. Do I need it to get the air out of the system so it does not overheat???