Have Volkswagen Questions? Ask an Expert Now.
Any good shop should have a "smoke machine" that uses nitrogen and a special fluid to create a fogging smoke inside the tanks and fuel system. They should either be able to detect if there are external leaks with this, or they can simply use the pressure test function of this machine to detect how much pressure drop there is- then spend some time pinching off lines until they figure out where EXACTLY the problem is, instead of just guessing. There are a number of hoses outside the tank. There are no hoses "inside the tank" that can register a leak- the tank is pressurized, and the test involves timing to see how long it holds pressure before the LDP needs to pump again. If it leaks internally in the tank, it's not LOSING pressure!!
Some common areas of leaks are the rubber seal for the fuel pump, the N80 purge valve sometimes gets carbon dust in it and doesn't seal exactly right, and sometimes the rollover valve gets a small leak past the diaphragm (which isn't typically available separate from the tank)