A method for dynamic pressure control during a multiphase injection is described wherein the pressures of fluids in the various reservoirs of a fluid delivery system are controlled to provide desired fluid delivery parameters. The methods include advancing the first drive member to expel the first fluid from the first reservoir into a conduit, wherein the fluid is pressurized to a first fluid pressure; measuring the first fluid pressure to provide a target value; while the second reservoir is in fluid isolation from the conduit, advancing or retracting the second drive member to increase or decrease the fluid pressure of the second fluid in the second reservoir to the target value; placing the second reservoir in fluid communication with the conduit; and advancing the second drive member to expel the second fluid from the second reservoir into the conduit.