I am not sure if there is a low pressure cut-off on this system or not, but if you only put in one can then it could be your don't have enough in there to build up the proper pressure.

Before you spend a ton of cash on this you might try just taking it to a shop for charging. They will have the correct equipment and know how even on a system as old as this one, and the cost will likely be far less than if you start replacing everything.