Leaking gas solenoid on MIG welder?

I thought I had looked at the gauge pretty recently and it showed that I had a fair amount of pressure left.

So- I am wondering if my solenoid gas valve may be leaking. How likely is this on a 4 year old MIG welder? Are there any things that can cause it to stick open that I should avoid? When I replenish my gas, how should I test to see if it is closing properly?

This all may be hysteria on my part- it probably just ran out of gas, but it just seems kinda sudden.

Have you tried pressuring the guage to say 30 Lbs. or so, shutting the valve and running a leak-down check? If excessive may also try a soapy mixture on all the fittings. My own will still holding within 5-10 lbs. after aprox. 15 years, in a half week period, when I forget, which has happened.