To seal the top cover of a vacuum travelling web filter, the top cover is lowered, by gravity, with a depending rim fitting against an upstanding rim of a filtrate chamber pan; the contaminant chamber above the filter web is closed off, and supply of filtrate is inhibited. Upon starting of the pump, a vacuum will result within the two chambers, drawing the cover tightly against the filtrate pan and, when a sufficient vacuum has been reached indicative of sealing engagement, supply of contaminant can be opened to draw contaminant into the upper chamber. As the filter cake builds up, further change in pressure in the contaminant chamber will result which, when a predetermined operating pressure level is reached, causes interruption of supply of contaminated fluid, and opening of an air vent valve to dry the resulting filter cake. Sealing fluid can be introduced between the engaging rims of the cover and the filtrate holding pan, between which rims the filter web is interposed, for example by introducing filtered fluid into a groove between the rims, or by introducing an inert gas. Preferably, filtered fluid is used which will be drawn back to the pump upon pump operation. A bypass line of filtered fluid to the pump prevents dry running thereof.