no,thats doesn't cause the pump to leak but leads to harder pumping as teh fuel only drips through the fueltap / petcock and a a lack of fuel while running the motor...on my 3bf the filter sits straight in the petcock,filterside upwards in the fuel (and the previous owner of my 3bf remounted it upside down, closed side above the filter-canvas showing to the inside of the petcock instead of away from it, so that it was a wonder that any had passed it anyways through the petcock)...
I just will have to check if i've got somewhere a photo for comparision of right and wrong mounted petcock-fuelfilters (guess i have such a photo but don't actually remember where i've stored it..)
Edit:Found the pic
2016-09-01-836 by
tomcatmj, auf Flickr
right hand the wrong assembled petcok,the right pipe was mounted upside down, the proper assemblage was done on the left hand petcock,where you can see the filter sitting on top of the plastictube and not inside the petcock as happened during the previous owners reassembling experiments to the right hand petcock...