I just got a nice tube to tow behind the boat and I had some questions about a possible air leak. The tube I have has a boston valve which makes it pretty easy to fill. My question is when I fill it up and put it on the ground and sit on it, it will lose some air around the valve but after it loses just a little bit it will stop and it wont leak any more. Is this some kind of feature that lets air out if you over inflate or is there something wrong with it?<br />Thanks