I have a 1984 Suntracker Pontoon boat that came with this pressure gauge. It's not hooked up. At the back of the gauge there is a place for a tube hookup, but no tube. Where on a motor does this tube get hooked into. I have recently received a 1977 150 Mercury and wonder if this pressure gauge can be hooked into this motor. But, I can't find a location to tie into. Any advice would be greatly appreciated. Thanks