A vacuum assembly for use in a fuel system of an aircraft, the vacuum assembly comprising: a housing ; a movable assembly positioned within the housing , the movable assembly including a first portion and a second portion , a first chamber being formed between the housing and the first portion , the first chamber being connected to a first fluid port and configured to receive a first fluid, a second chamber being formed between the housing and the second portion , the second chamber being connected to a second fluid port and configured to receive a second fluid, and a third chamber being formed between the housing and part of the movable assembly between the first portion and the second portion , the third chamber being connected to a third fluid port and configured to receive a third fluid, wherein the movable assembly is configured to move within the housing in response to pressure generated by said fluids communicated to each of said chambers via said ports ; and a biasing mechanism coupling the movable assembly to the housing , wherein the movable assembly is movable between a first position and a second position within the housing by the pressure generated by the fluid communicated to the third chamber being greater than the pressure generated by the fluid communicated to the second chamber , and overcoming the biasing force of the biasing mechanism as a result, to create a low pressure area between the housing and a first portion of the movable assembly ; characterized by the movable assembly further comprising a fluid flow path fluidly coupling the first chamber and the second chamber ; and a valve disposed within the fluid flow path and operable to selectively expel the first fluid from the first chamber into the second chamber . a housing ; a movable assembly positioned within the housing , the movable assembly including a first portion and a second portion , a first chamber being formed between the housing and the first portion , the first chamber being connected to a first fluid port and configured to receive a first fluid, a second chamber being formed between the housing and the second portion , the second chamber being connected to a second fluid port and configured to receive a second fluid, and a third chamber being formed between the housing and part of the movable assembly between the first portion and the second portion , the third chamber being connected to a third fluid port and configured to receive a third fluid, wherein the movable assembly is configured to move within the housing in response to pressure generated by said fluids communicated to each of said chambers via said ports ; and a biasing mechanism coupling the movable assembly to the housing , wherein the movable assembly is movable between a first position and a second position within the housing by the pressure generated by the fluid communicated to the third chamber being greater than the pressure generated by the fluid communicated to the second chamber , and overcoming the biasing force of the biasing mechanism as a result, to create a low pressure area between the housing and a first portion of the movable assembly ; characterized by the movable assembly further comprising a fluid flow path fluidly coupling the first chamber and the second chamber ; and a valve disposed within the fluid flow path and operable to selectively expel the first fluid from the first chamber into the second chamber . <EOS>