Strawberry milkshake in hurth transmission
 

I have an older searay with twin 340 hp mercs inboards with hurth transmissions. One transmission is full of strawberry milkshake water and ATF. I pulled the tranny cooler and pressurized the oil side to 20 psi with air no leaks…I’m thinking maybe when it gets hot it opens and leaks? Can I get water in tranny anywhere else? Should I just go with new cooler….