RE: Jett .60LX and sundowner 50.
Only two fuel tubes are involved - feed and vent. No need for a check valve of any kind.
Feed line from the tank meets at the joiner and then continues to the engine.
The best way to fuel the bubble-jett tank is by using the jett tanker.
Proceedure is to disconnect the feed line coming from the tank (either at the engine or at a joiner as I have shown), connect the tanker, and use the tanker to remove as much air from the tank as possible (suck the air out). This just gets things started a bit.
(note, if you are fueling with a smaller syring or other device, yes..... at this point you want to clamp off the feed line with a clip or hemostat to prevent air from entering the tank again)
Then fill the tanker about 3/4 of the way full of fuel.
Reconnect to the feed line, then hold the tanker upright, and draw out on the plunger to ensure all of the air is out of the tank (will suck out bubbles and then meet resistance). Any air will rise to the top of the fuel inside of the tanker. Result is you extract air then add fuel in one operation.
Keep the tanker upright and slowly push down on the plunger to fill the tank. When resistance is felt, stop, and slightly pull out on the plunger. The tank is full. Usually at this point I clip a hemostat on the feed line to keep it from spraying fuel at me when I reconnect the joiner. And I ususually leave the hemostat on until I am ready to start the engine.
The tank bladder does not pressurise the system... does not stretch ... but sometimes after fueling it does hold a little pressure to let fuel squirt back at you. So use a little caution.
The operation of the bubble tank is dependant on getting all of the air out first. A syringe or the jett tanker does a good job of this.