It's all been covered above but I'll just recap. The primer draws fuel in the angled barb and spits it out through the straight barb. You can put a finger over those fittings to confirm. It will only draw in one and only spit air out of one while you hold it in your hand to test.
Those primers are crap now and can't be trusted. The old ones would last five years at least, now they spit gas at me while I'm pumping them in the first year sometimes. WFO noticed this a couple years ago and I saw it myself soon after. It might be ethanol but I think another Chinese company just got the contract that has lesser quality control. The racers have moved to these button primers
http://www.ebay.com/itm/PRIMER-KIT-...ash=item2362e5139f:g:qVEAAOSwB4NWv8yY&vxp=mtr I've installed my first one in my black XPL and it works pretty good.
Since we know the traditional primer will fail you want to keep it off your supply line. If you run on the ON channel and only use the RES for emergencies then the RES is a good place. Otherwise I do it like 1 of 500 said (originally taught this by Crockett on pwctoday may he RIP) I loop the return line and put my T in the bottom of the loop. That way there is always a few squirts worth of fuel there and there is zero risk of loosing an engine from the primer drawing in air. I tried a low spot in the return line at first on the Zebra ski and like Suke I was never able to get any fuel pumping when the ski had been sitting (when you need it most haha).