this is my understanding of this system. maybe i got it wrong. please correct me if so.
if the flow meter zeros...which it did...there is no leak. further smoke testing is not required. there should be no vac or pressure build up on these builds as the evap vent is always open. the system is only sealed during leak testing. pressure and vac can only occur due to a clogged or faulty evap vent path. I've seen tanks collapse bend sideways and hit the driveshaft due to clogged vent paths...yeah fuel pumps can suck hard.

 

You are pretty much correct. The vent valve is commanded shut and the purge valve is commanded to open a certain percentage via pulse width modulation with the engine running. which puts vacuum in the tank (removes air) it holds the vacuum for a period of time then opens the vent valve to release the vacuum. when its shut off the vent valve can shut again for the second half of the test. its shut for another 15- 20 minutes. if it passes the degradation test part you won't get the light or message. I have done the "service bay test" and had it pass just to have the light come back on the next day due to a line that was stressed or an "o"ring in a connector fail. which is the point of dropping the tank and capping the openings and flow/ smoke testing it. The pulse width modulation of the vent or purge valve can be an issue as it doesn't open close valves with as much force as straight voltage. I have also found cracks in the charcoal canister which can remove the smoke from the leak. which is why the flow test is a good test too. you do definately have to make sure any plugs or caps that you put on are removed to avoid collapsing the tank for sure. the large leak test is interesting to watch but the small leak (degradation) part is like watching paint dry.

 

As far as leak testing goes, yes, the meter going to zero indicates no leak - but you have to keep in mind that the EVAP system is under pressure when smoke/flow testing is done. Under normal operating conditions, the system is under vacuum when the purge valve is opened, and it is possible for a leak to be present under vacuum but not under pressure. As the PCM commands the purge valve open, it expects to see a proportional change in tank vacuum - if that change is less than expected, the gas cap message (which is also the same as a large leak code P0455) will come on.

In this case, we don’t know for sure what the freeze frame data for the code was, nor do we know if the OP’s mechanic attempted to run the smoke test under the same conditions. It could be as simple as the ambient temperature causing the gas & tank to expand, during which time the leak is present; but the smoke testing isn’t done until the next morning, and by then, everything has cooled off and the leak is temporarily sealed again.

According to the OP, the gas cap message has been coming on for 2 years - I can understand why it would be easy to look at other things since the vent valve fixed it the first time, but gas caps are as prone to failure as any other component. They are a lot cheaper than a vent or purge valve and given that both have been replaced, in addition to the fuel pump, is it beyond reason to suspect it has failed as well?

I’ve also seen gas tanks collapse due to clogged EVAP systems. The most common failure we see is the vent valves getting clogged with dirt (many of our customers live on gravel) and failing to work at all. Usually by the time we get to them, dirt has gotten into and/or through the canisters. Every time we replace a valve we relocate the fresh air filter to the engine bay, because GM’s recommendation to position it above the transmission wouldn’t keep them from getting plugged.

 

New OEM cap.

 

I've owned the truck for a little over 2 years. The P0455 code came up shortly after purchase. A new vent solenoid fixed it. The fuel cap message started coming up about 2 months ago. I've replaced the cap, purge valve, and vent solenoid. After the first leak test found a leak at the top of the tank, I replaced the fuel pump due to a pin hole from rust and all of the tests I've listed have been done since then. The fuel cap message problem still persists.

 

The PCM runs the large leak test once per cold start as long as the correct operating conditions are met. The PCM is basically just looking for vacuum to be maintained within the fuel tank. The only causes for this code are a large leak, the FTP sensor, PCM, or related wiring.

If your mechanic didn’t perform the smoke testing under the same conditions which the code set (engine speed and temp and intake air temp are the primary factors), that’s when the test needs to be done. Temperature is going to have a significant influence because the cause of the leak could be affected by temperature.

Keep in mind that new components does not mean good components. Failures happen with new components for a variety of reasons - do not assume the parts you’ve replaced are good just because they’re new.