A method of modifying an existing valve assembly, comprising the steps of (a) providing an existing valve assembly having an existing sleeve with an inlet window and an outlet window and a piston having an opening for selectively communicating the inlet window with the outlet window; (b) providing a housing having a housing bore, wherein the housing has a housing inlet window and a housing outlet window, (c) placing a supplemental sleeve within the housing bore,wherein said supplemental sleeve has a supplemental sleeve inlet window and supplemental sleeve outlet window; and (d) placing the existing valve assembly, having the existing sleeve within said supplemental sleeve, and the piston opening selectively communicating the housing inlet window, via the supplemental sleeve, with the inlet window of said existing sleeve to the outlet window of said existing sleeve and then said supplemental sleeve outlet window and said housing outlet window, and said piston being moveable along an axial direction; wherein said existing valve assembly has an existing flow volume and pressure, and said supplemental sleeve being designed to change at least one of said flow volume and pressure; wherein at least one of the supplemental sleeve inlet window and the supplemental sleeve outlet window has a different size from the existing sleeve inlet window and the existing sleeve outlet window to achieve the change in at least one of said flow volume and pressure; and wherein said existing valve assembly is part of a spool valve in an engine fuel supply system. (a) providing an existing valve assembly having an existing sleeve with an inlet window and an outlet window and a piston having an opening for selectively communicating the inlet window with the outlet window; (b) providing a housing having a housing bore, wherein the housing has a housing inlet window and a housing outlet window, (c) placing a supplemental sleeve within the housing bore,wherein said supplemental sleeve has a supplemental sleeve inlet window and supplemental sleeve outlet window; and (d) placing the existing valve assembly, having the existing sleeve within said supplemental sleeve, and the piston opening selectively communicating the housing inlet window, via the supplemental sleeve, with the inlet window of said existing sleeve to the outlet window of said existing sleeve and then said supplemental sleeve outlet window and said housing outlet window, and said piston being moveable along an axial direction; wherein said existing valve assembly has an existing flow volume and pressure, and said supplemental sleeve being designed to change at least one of said flow volume and pressure; wherein at least one of the supplemental sleeve inlet window and the supplemental sleeve outlet window has a different size from the existing sleeve inlet window and the existing sleeve outlet window to achieve the change in at least one of said flow volume and pressure; and wherein said existing valve assembly is part of a spool valve in an engine fuel supply system. wherein said existing valve assembly has an existing flow volume and pressure, and said supplemental sleeve being designed to change at least one of said flow volume and pressure; wherein at least one of the supplemental sleeve inlet window and the supplemental sleeve outlet window has a different size from the existing sleeve inlet window and the existing sleeve outlet window to achieve the change in at least one of said flow volume and pressure; and wherein said existing valve assembly is part of a spool valve in an engine fuel supply system.