Coolant not reaching the rear cylinder
New here. I just recently acquired a 1987 VN750 and decided to do a radiator flush and oil change (the previous owner said "I don't know" when asked about the last time he did it.) oil change went off without a hitch. The coolant flush not so much. When draining the system I got coolant coming out of everywhere except the rear cylinder head. Weird. Closed the drain plugs Added some more coolant. Checked that drain again. Nothing. Ran the engine for a bit. Checked it again. Nothing.
I'm at a loss. Any help will be appreciated.
(To answer as many questions as possible prior to posting I will add that it still runs. And other than melting my manlihood in hot weather traffic it doesn't really overheat)