Convenient for you I can actually go into more detail with pictures! I currently have a F600 on deck starting assembly today. Which is essentially is the same design for the f500 bottom. So this is the gasket Im talking about-when is your next voyage to southeastern Wisconsin?
When applied sloppily, and not working it fully around the corners, when you mount it, it will fold the gasket under one side or the other, instead of compressing between the pieces, creating the air tight seal for the housing. I check every single one I do as soon as I install it, and it even happens to me sometimes and I have to take it back off and reapply the gasket and remount it. So Idk if its something they are double checking on the assembly line. Here is a photo of it mounted(not tightened down yet) with a flash light from
The outside and you can see the light coming through-
Here it is tightened down you see it set perfect, no light. Only light you see is the stuff from the shop lights hitting the wheels Im working on. But the seal I pointed out is perfect. And a second picture of what a perfect seal will look like with a flashlight on the inside-
And finally, if that all checks out, maybe someone forgot the proper amount of cement around the double bottom for the ash pan, to check this shine a flashlight through the big hole on the back bottom towards the ash pan and see if you see any light coming out between the cracks of the double bottom on the ash pan. This is the cement they may have slacked on-
And if it was done right, you see this awesome overflow when you tighten the whole bottom together. (The overflow stuff does break off eventually, all the important stuff is in the actual joints)