Whenever you suspect a leak, try something simple to find it. It takes almost no time to do the tests.
If you have a fuel bulb or one of those big plastic syringes you're good to go.

Unhook the fuel tubes from their ends. Unhook the muffler pressure line from the muffler, the delivery line from the needle valve, and the fill/vent line from it's fuel dot or pull it's plug. After all, you want to test the lines and their connections as well as the tank, right.

If you have a three line setup, the connect two lines to each other and connect your bulb or syringe to the 3rd line. Put some pressure on the bulb/syringe and hold it. While waiting to see if the pressure is leaking, listen carefully. Most leaks will show up under this simple test, but some don't respond to pressure. Those will often respond to "vacuum". So introduce vacuum into the test. If you have a syringe, pull it's plunger until you've got a decent "vacuum" in the system and hold that. If you have a bulb, then squeeze it flat and connect it to the 3rd line and it'll "vacuum" for you.

If you've got a two line setup.... plug one line and blow/suck on the other one.

It's dead simple to test stuff like this. Testing beats discussion every time for finding a problem.

BTW, if you're using muffler pressure, then the system already is seeing an introduction of "extra air" into the tank. For the bubbles to be "air" coming from somewhere else would suggest that there is suction greater than the muffler pressure that's available at the point of the leak. It's worthwhile to check your muffler pressure into the tank to make sure it's got a clear path. Muffler pressure feed lines sometime plug and that causes problems. Check it out.

Also, a bunch of today's engines are really designed to run with muffler pressure. If you're not using it, it's never a bad idea to start. Some of the more powerful engines can run without it, but won't do as well since their venturi areas aren't optimised for "suction".