You can blow them up to a much larger diameter than that of a tire and they should withstand that. When the tube is under water wipe off any bubbles stuck to the tube when you were submerging it. Look whether any bubble springs back to life. Turn the tube to make sure you view all of its submerged surface. Are the valves Schraeder or Presta? If Schraeder you can move your finger over the top of the valve to release air trapped in the outer part of the valve. If that space fills up with air again, the valve is leaking. Internally the valves have thin rubber washers and that rubber can rot. The Schrader valves normally have cores that you can replace and Presta sometimes too. If a valve is leaking and the core cannot be replaced or the leak persists after changing the core, toss the tube.
true, I did most of that. I brushed some bubbles off that were stuck to the tube but weren't bubbles from leaks. I flipped the tube so the valves were pointing outward and checked both sides of the tubes. No leaks. They are schreader vavles but I didn't press the valve while it was underwater - I thought maybe that could cause a problem but I did do that to one and it didn't leak after.
It's good news that so many of you say it's normal for them to slowly go flat over a few weeks or especially months. I just thought you know, they were air tight. so the air escapes through the actual rubber and/or the o rings on the valve? Doesn't really matter though. Strange though is that I have yard machines with pneumatic tubes that don't seem to go flat over after years. I guess I'll inflate them a bit more though and check psi.
thanks for the input. I'm in NY and it's been really hot heat wave like 95 degrees for days. I didn't know tubes aren't air tight (over weeks/months) and they haven't been inflated since last Fall probably. I just assumed they were flat. And yes they are probably the cheapest thinnest tubes on these cheap bikes.
I did over inflate and put in a bucket or water and paid special attention to the valves but didn't see a single bubble. I would imagine a bucket of water works better than soapy water put on the tube. a lot cleaner too.