The gas cap needs to be vented for sure. It may be just a case of cleaning the vent port on the old one? You may not need a new one. But even more importantly: Why was the crankcase full of gas? I have a hard time seeing how a non-venting gas cap could be causing that much fuel to get in the case. If it's building pressure, that could overpower the float. But the only way I can see it creating substantial pressure is when it gets hot. Which might be the case if it was sitting in the sun and was quite cool the last time the tank was opened. But (and I could be wrong) I have a hard time believing it would create enough pressure to force enough gas into the case to fill it above cylinder level. I would think the pressure in the tank would have equalized loooong before it dumped that much gas into the case. It sounds more like two separate problems: i) Gas is free-flowing into the case somehow. ii) The tank is not venting properly.
I confess: I've worked around a few 1650's & 1655's (gas & diesel), but only one 2-70, and it was a diesel. Did the 2-70 gas have a lift pump like the 1655's gassers and 2-70 diesel did? If so, that needs to be checked. The diaphragms in those lift pumps go bad and allow gas to pour into the crankcase. But from what I can tell in my manuals, the stock 2-70 gassers were just gravity fed straight into the carburetor. If that's the case it sounds like just a sticking float/needle. Whatever the case, the fuel in the crankcase problem should be addressed. Has someone added a fuel pump to your gasser? I doubt it, but I recall hearing of some IH hillside mods where IH dealers and owners added a mechanical fuel pump to tractors and combines that were typically gravity fed - to make sure the carb was always fed fuel, even on a slope where the tank level might be below the carb.
Are you sure it was pressure, and not a vacuum? A 2-70 gasser would be a thirsty unit. Even if some pressure could be building due to the tank getting hot from running and/or being in the sun, I would think that it would be using gas at a rate that would prevent it building any pressure. Most (99%) of symptoms due to a plugged tank vent result in it creating a vacuum, not building pressure. Which may also be caused if too much fuel was free-flowing flowing (from, say, stuck needle valve or bad pump diaphragm, which would explain the gas in the case). The fact that gas sloshed out doesn't necessarily mean it was building pressure instead of creating a vacuum: When fuel tanks build vacuum, the vapour pressure goes down. When you relieve it, wacky things can occur: Vaporized fuel will do dense rapidly and it will have a large inrush of air to equalize the pressure, and this causes fuel to slosh everywhere.
This is definitely speculation and guesswork on my part. But I'd be surprised that you'd build pressure in the tank of a running 2-70 gasser due to an un-vented cap. I'd think it more probably you created a vacuum, as that's the most common problem with a plugged cap vent (and because a 70 HP gasser will use fuel at a pretty substantial rate). Although I've never been around a gasser 2-70, I own a gasser Allis 180 - exactly the same engine displacement. When I first got it and it was acting up, it was due to exactly that: Plugged cap vent creating a vacuum in the tank.
I'd get a vented cap on there. But before running any more, I'd also be trying to figure out why gas was getting into the case. Even if my thoughts on vacuum vs. pressure are wrong, I just can't see how pressure alone would fill the case that full of gas. Sounds to me like either a stuck needle valve or bad pump diaphragm (if it has a pump, which I don't think it does). And a stuck needle valve (and/or bad pump diaphragm) definitely need to be fixed, as they won't do any favours for your engine.
