Hi all,
The great things about "Eheim Classics" are that you can get spares for them, and they are very simple. They just have one gasket (between the head and filter body), and the water goes in the bottom and out of the top, having passed through the filter media. They don't have a a priming button, but you don't need one, just suck on the outlet hose to get the water flowing under gravity into the filter, and when the water reaches the outlet hose turn the filter on, rock it backwards and forwards a couple of times (to purge any air) and you're done. I have an inlet sponge and ceramic/glass rings in the filter, this way as long as you clean the inlet sponge/exposed hoses up every couple of weeks you can leave the filter un-opened for months at a time.
I like to have double taps on the hoses (for ease of cleaning), but you don't need them.
cheers Darrel