Pressure in fuel tank

I just put my tank back after removing and totally dismantling it to coat the inside with Epoxy.
Now there's a significant 'whoosh' when I open the fill cap.
I don't think its normal, but this is my first Florida Summer with the bike, so I'm not sure how much pressure should build-up.

The bike is a 2008 US-version GRiSO 1100/2V
I have previously removed the Charcoal vapor canister, and joined the TB ports together with a hose.
I checked the 2 vent hoses at the back of the tank, and they are clear.

Questions:
Is this normal ??
Is there another internal vent in the tank that I might have clogged with epoxy ??
The two vents seem to go up through rigid plastic tubes, and exit in the fill-cap area. Is there a critical way to align the fuel tank surround or gasket or filler cap ?

And - did I do all this for nothing ??    
I used Caswell 'Dragons Blood' epoxy inside the tank.  We have issues with Ethanol gas here, especially in a hot climate like Florida.
I haven't heard of many issues with the Acerbis tank on Guzzi's, but I got bitten badly by this when I had
my Ducati Sports Classic. Some of those are on their 3rd or 4th tank......I only had 2.  

Jeremy

I am also getting the whoosh when i open my fuel cap. Once blew some gas out of the tank.
This is a new bike with less than 500 miles on it. Will ask the dealer service what goes. The tank should vent better than that.

There was a short run of tanks that had blocked vents, there was a 'Technical Update' campaign for them from memory. I think it was the vent rather than the drain. I'll have a look later.

Pete

here you go...

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Thanks Phang.

Pete

Thanks Gentlemen.
I have a 2008 1100/2V, so probably not covered by that Service Bulletin.
And, its not currently doing this - it 'fixed itself'.        Hmmmmmm

Put compressed air down the vents anyway, and everything is clear.

Jeremy

My dealer fixed my tank by finding a pinched hose. It is blissfully silent when I open the gas cap now.

Well, Bugger me, it's started doing this again - and worse...
Came out to the bike yesterday morning, and a there was a large 'dent' in the tank. I was PISSED
However, opening the gas cap released a lot of vacuum, and the dent pretty much went away. I didn't know painted plastic was that flexible...
Anyway, I obviously have a venting issue. Something (crankcase vacuum after shutdown, temperature changes ??) is generating a strong vacuum.

I removed the gas cap completely, and blew thru the 2 vents both ways and they were clear.

I suspect the problem is with this contraption inside the filler neck:

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So, has anyone worked-on or had issues with this ??
(For the ever helpful Aussie Gurus - you may not have this thing. My bike is a US-spec 2008 1100, and this looks like some emissions-related crap)

This part of the filler neck covers the vent tube (I think) under this cover is a ball check-valve. I think its to allow over-pressure venting, but not spill in a tip--over ?
The Moto Guzzi Workshop Manual and Parts List is, as always, f-ing useless. It shows the tank cap as one complete unassembled item.
The Guzzi Engineers really should invite the Technical Writers to the pasta lunches where all this stuff is designed and discussed. Then they'll know WTF some of the parts on the bike actually do....

Global Weirding!