Eheim External Wet/dry Filter

Does anyone use or have any experience with a wet/dry filter , one of my second hand tank purchases from ebay  came with a spare external filter a eheim 2327 wet dry filter the owner threw it in with the sale as spares or repairs because he had became fed up of not being able to get it working properly on his tank, it been sat in my even expanding pile of spare parts and bits for the last few months , so i decided to have a look at it today , so after a searching for as much information as i could on the internet (not much available ) i did find a very basic manual that seemed a bit lacking in its information i set it up to run on a spare tank in the garage.
i started by priming it all up and switched it on and at first i thought great all seems to be working , that was untill the air/float part ran out of water at which point everything seemed to stop no water in no water out, after trying various ways i found i could get it to run non stop if i put the breather pipe in the water but this then means that it sucks water not air and the float never drops , leading  me to the conclusion that the water is not being bypased somewere when its in a air cycle.
Now iam one of these people that hates to be beaten by things i cant work out so iam currently planning to strip the head fully down and see what's what and what makes it tick etc but any help would be gratefully received and also what's the advantage or disadvantage to a wet/dry filter

Thank you steve , i did find this link but quickly dismissed it as all in german  ,
Yes i think that's how i understand it now as not running non stop (seems a little weird ), i am not sure even if i could get it working that i would use as they seem to be noisy aswell but i would like to figure it out just for the satisfaction and as a emergency back up or possible resale , i read somewere that one of the common causes for these to not work is a air leak some were so my plan is a full strip down and renew al the o rings (of which there are many and i also noticed that the float switch seems to have some slight damage so will order a new one of these and try again.