Where can i find the air dryer filter on my Ford cargo 1313?

Hi,
I have an Ford cargo 1313 1985. Since my last travel the air vaults don't fill, been having some serious rain.
I emptied the vaults using the pins, but the air still doesn't come up.
So now i guess the air drying filter is not working anymore, but i cannot find it:(
Found the oil filter on the bottom of the engine, the fuel filters on top of the engine.

Someone here who can help me?
I'm from Holland, so excuse me for my poor English.

Greets Bruce