Is the waterlevel in the fishtank higher or lower than the level in the filterbox? (I mean when you hang it on the back, your tank-water level should be slightly higher or equal to the maximum filling level of the filter box. If it is lower, then of course the syphon will go the wrong way.
Water will try to level itself, so by topping up the tank to a higher level, eventually the syphon will start in the correct direction).
Good luck, very nice find and amazing good condition.
I was just typing something similar to that... Thanks,,,,

. With some (not all) you needed to strap them down or the filter would almost fly off the Tank. And you young ones like to b???? about noise from the new HOB's!!!! I posted this before but some needed to have you put your thumb over the intake tube submurge the intake tube into the take drain the air and as quick as you could put it into the filter, if you good at it ....maybe three tries, but you could spend a half hour priming the pump....This is Kewl.......Thanks!!!
. Supreme was very popular and came in different models again some of the later ones had a early version of what we call today a auto prime. Guppies could get sucked into the filter and many times get spit out without a scratch!!!!