A device for injecting a therapeutic agent into an ocular implant at least partially implanted in an eye including an injection lumen providing a pathway for injecting the therapeutic agent into the implant; an outlet lumen providing a pathway for pre-existing fluid in the ocular implant to exit the implant; and a collection chamber fluidly coupled to the outlet lumen that provides a first fluid outflow resistance and a second fluid outflow resistance. The first fluid outflow resistance is lower than a first resistance to outflow of the implant. The second fluid outflow resistance is greater than a force imparted onto the implant by intraocular pressure of the eye. Injection of therapeutic agent into the implant via the injection lumen causes the pre-existing fluid to exit the implant and enter the collection chamber via the outlet lumen and causes a second pre-existing fluid to displace from the collection chamber.