A fluid transfer device includes a housing and a reciprocating piston in the housing. The piston and the housing may be configured to at least partially define at least two variable-volume chambers. An inlet chamber port and an outlet chamber port may be associated with each of the at least two chambers. A valve member may be operatively associated with each inlet chamber port and outlet chamber port to permit or prohibit fluid flow past the respective inlet chamber port and outlet chamber port. The two chambers may be in a state of fluid isolation, such that fluid in one of the chambers does not enter into any other chamber. A valve actuating means may be configured to shift the valve members to permit or prohibit fluid flow past the inlet and outlet chamber ports. A release mechanism may be associated with at least one valve member. The release mechanism may be configured to allow the valve actuating means to shift the valve members to permit or prohibit fluid flow past the inlet or outlet chamber ports. A sub-system in a post-mix beverage dispenser is also disclosed using a fluid powered proportioning pump.