leaking engine antifreeze at this white plastic connection may make your engine run hot . so this plastic fitting needs to be replaced with brass ASAP. when you do that you may lose antifreeze out of the engine -to - hwh hose. have a helper ready to hold this hose and close it off ( harbor frieght carries a nice set of u shaped threaded hose shut off clamps. ) have helper hold hose as high up as possible to minimize antifreeze loss as you undo that plastic connection. ( you may also lose antifreeze from the heater coil antifreeze out put line .) if you install an air bleed valve in the antifreeze input line, have the valve pointing 'up' not off to the side. then you can add antifreeze by gravity with a funnel and hose. running your engine with an air lock in the engine to hwh tank is not good. when i changed out both of my engine to hwh hoses i decided to change out all the antifreeze in the system with fresh, since i was going to lose a lot in the heater hose replacement process any how. (i had to replace my heater hoses at 5 years, because there was inadequate anti chafe on the heater feed hoses from the engine block, which caused pinhole leaks in the hoses detectable as a loss of antifreeze only at high engine rpms and operating temps ...( this issue was detectable only by using a STANT tester....)