Hope this doesn't sound too pedantic, but I would suggest you to change the whole approach of the algorithm. Since this approach really needs a lot of tunning every time you add a new operator. Even more so, as operators go getting more complicated, your method will become extremely inpractical.
I believe a more correct solution, yet that requires a bit more work, is to actually parse your infix expression and build the binary expression tree out of it.
This shouldn't be all that hard since grammars for arithmetic expressions are defined all over the internet. After you've found a grammar that suits your needs, perform the parsing, build the tree, and when you have that you can get your postfix notation by performing a post-order traversal of the tree.
The Wikipedia article on binary expression trees may be a good place to start your documentation on this subject. Hope this helps!