How are floor jacks rated?

[newlywv]

I have been in the market to upgrade a duralast aluminum floor jack. It is rated at 3 tons, but can barely lift one side of the front of a 1/2 ton truck. I have noticed other larger jacks that are made of steel and 4 times heavier, and still 3 ton rating. I am somewhat confused on the rating system, if there is one. Any suggestions?
Thanks,
Rich

 

[ajchien]

I hated the duralast (Larin) 2.5ton SUV jack I had. I had to constantly purge air out of the system, and it incessantly leaked oil.

I'd say follow the trouble shooting guide that came with it to see if you need to purge air out of it.