Usually the larger 3/8" line is for the V6 engines and higher. No problem with using 3/8" on your 100. The only tanks I know that have a check valve are the built-in tanks that use an anti-siphon valve. Most of the cheaper Tempo tanks don't have a check valve. The fuel hose has two ball valves in the hose bulb. Usually these last a long time, but I've seen where one of the ball valves will fail, requiring replacement of the in line whole ball valve with a new one. Biggest problem I've encountered on these old plastic tanks is where the tank pickup gets a hole in it somewhere. Fuel below the hole won't get sucked up. Also, many times the plastic pickup is just pushed onto the tank head hose barb. With years of use, that seal where the fuel pickup meets the tank head will loosen up and permit air to get sucked up instead of fuel. You should be able to pump up the fuel hose bulb (hard) first thing each day. Once the engine is running, and the pump is sucking fuel, the bulb will soften and will run fine that way the rest of the day.