Half-sleeved and sleeveless plastic piston pumps

A brake pump assembly for pressurizing a fluid comprising:

• a block defining a piston bore,
• a piston slidably disposed in said piston bore including a trailing neck portion and defining a primary pump cavity on one side of said piston,
• a low pressure seal disposed between said piston and said piston bore,
• a first sealing lip integral with and projecting longitudinally from said piston toward said primary pump cavity and spaced from said low pressure seal and sealed to said piston bore to define a secondary pump cavity between said trailing neck portion and said first sealing lip and for flexing radially inwardly in response to a positive pressure differential between the fluid in said secondary pump cavity and the fluid in said primary pump cavity to allow fluid to flow from said secondary pump cavity to said primary pump cavity,
• said first sealing lip extending radially outwardly from said piston for biasing said sealing lip radially toward said piston bore in response to a negative pressure difference between the fluid in said secondary pump cavity and the fluid in said primary pump cavity, and
• a second sealing lip integral with and projecting longitudinally from said trailing neck portion of said piston toward said secondary pump cavity wherein said second sealing lip extends radially outwardly from said trailing neck portion of said piston for sealing engagement with said piston bore.