My diesel engine has been drawing in air since I had it serviced at 126000 miles causing all the normal associated problems. The mechanic,who carried out the service at first thought it was a badly seated filter but then told me it was a missing core plug in the filter housing which he replaced and liquid metal-ed over to prevent it working loose again. However the fault reappeared one week later and having more time I took a look myself. I noticed at the back of the filter housing, close to the top, is an orifice blanked off with a core plug on the drivers side and on the passenger side, under the large dollop of liquid metal applied by the mechanic, I found not a core plug, as I had been informed, but some sort of a 'valve'. This has a plastic body with the tip of a stainless steel pin poking out about 1/8". I can push the pin into the plastic body against a spring pressure and by gripping the pin with a pair of pliers it can be completely withdrawn from the housing. The pin is about 1" long and 1/16 in diameter. When I operate the fuel priming pump diesel fuel is emitted from the join between the housing and the pin; so presumably air can also be drawn in here. What is this 'valve'? and can anyone venture an opinion as to what could have caused it to fail? bearing in mind the car had just been serviced! The top of the filter housing is stamped with the numbers 4, 2, 5.The engine is a TUD 5 1500cc diesel unit.