There were basically 2 piping diagrams for these solar units. Grumman was known for its aluminum solar panel construction. One piping scheme used a solenoid valve to dump the panel water during freeeze-possibilities. These systems failed in about one year.
The other piping used a heat exchanger between the domestic water tank water and the antifreeze solar loop. Water pressure pushed water through the tank on the domestic water side. A pump moved water throught the panels and heat exchanger (which might have been a loop of copper inside the tank.) This is the most likely piping arrangement that would last 18 years.
No matter what the piping diagram, your tank leaks and is no longer usable. It needs to be replaced. Triangle Tube makes stainless steel heat-exchanger domestic water tanks that will replace your old one. Their tanks are also sold as Weil McLain Gold tanks. Superstore is another manufacturer. Call the local plumbing supply and ask for a good quality, boiler-fired water-heater tank, or conventional electric water heater and heat-exchanger combination that they can get.