I have merc trans and was curious why when I disconnect my cooler lines from the trans coolers transmission fluid leaks out. Seems to me that pumping water into a line that has trans fluid coming out would just fill the trans with water. Any input would be greatly appreciated.

your trans cooler is probly cracked inside the tranny. all you have to do is remove the 4 bolts and pull the cooler out if its cracked all you have to do is solder it instead of paying $300 for a new one.