That sounds like part of the hot gas defrost system. Possibly a strainer or check valve. Follow the line you called "suction" and see where it goes. The case may have been field converted to electric defrost and that part may not be in use.
"You do not merely want to be considered just the best of the best. You want to be considered the only ones who do what you do.".... Jerry Garcia
"You just have to decide if you're a Tigger or an Eeyore,..I think you know where I stand on the great Tigger/Eeyore debate." Prof. Randy Pausch