I think your customs are brilliant FlatCat!!!
Maybe if you can take off the pump from the top of the fire engine then pumped water through it where it wont get your Playmobil wet you could find out what is wrong.
Maybe it is not tight enough?
I am sorry, I do not know much about fire engines,
And I dont have any so I don't really know how to help, but that is my suggestion.
Hope you get it working.