Problem with closed loop solar hot water system

[Ruth58]

I have a closed loop solar water heating system with two panels. The expansion tank recently failed and I have replaced it along with recharging the system with a 50/50 mixture of antifreeze and distilled water. I also replaced the pressure relief valve. My problem since then has been the pressure relief valve has released a large amount of the antifreeze solution and now I don’t have any PSI on the gauge. The system had never done this before, so does anyone know what could be causing the pressure to get so high causing the relief valve to have to expel fluid? The controller seems to be working, but I am not sure if I need to have the switches set differently. Do I need a specialized pressure relief valve and what should it be rated for? The one I bought as a replacement is rated for 75 PSI. Any help would be appreciated.