for the negative and positive pressure relief the end cap 106 carries a centrally mounted pin 134 the head or distal portion of which 136 is tapered and dimensioned to fit into the orifice 118 and restrict its cross sectional area the body portion of 138 of the end cap 106 is made of a relatively thinner plastics material so that it can deflect resiliently under the influence of the pressure differential between atmosphere outside the device and the negative pressure prevailing in the chamber 112 the cross sections of the orifice 118 and needle 136 are tailored in relation to the flexibility of the body portion or diaphragm 138 such that within an expected pressure range movement of the needle 136 into the orifice 118 will control the pressure in the chamber 110 as well as the flow rate through orifice 118 to be substantially constant it has been found that the diameter of the orifice 118 should be between 3 and 8 mm while the needle 136 on maximum insertion into the orifice 118 should decrease its cross sectional area by 20 to 50 of its free cross sectional area e g decrease by 30 for a pressure change of 40 to 80 kpa it has also been found that.