what a good air tank,i have had to return two tank because they would not hole a charge long.
Why do you need a portable air tank?
How long is “long?” How long do you expect a portable air tank to hold pressure?
Every portable air tank (mechanical/automotive applications) I’ve ever used couldn’t hold a charge for more than a day at most. They aren’t meant for long term storage.
I bought a yellow one at Tractor Supply Co for $15 about six months ago. It was made in China and has a date stamped into one of the legs after which I’m supposed to dispose of it. It has a shut off on same valve where I put the air in. I’ve seen it hold a full charge for up to a week.
Sometimes the connections aren’t tight enough. Try disconnecting them and using antisieze tape or thread sealer on all connections. It is usually necesary to use something on the threads to avoid overtightening. Leak test with soapy water on the OUTSIDE of all connections.
Almost all inexpensive tanks are made in China and almost all of them are OK.
Find the leak and FIX it! Spray the fittings with soapy water. It’s usually the safety valve or the quick disconnect fitting.
Do what I did. Throw the crummy original shutoff valve away and replace it with a natural gas valve.
It seems to work very well. I can air it up and a month later it will still be holding with no noticeable loss of air pressure at all.