According to embodiments, priming systems, methods, and devices are disclosed which allow medical treatment devices which pump fluid to be primed with minimal operator intervention and a high level of convenience. A blood circuit with a filter fitted with one or more air vents on a non-blood compartment is attached to a treatment system and priming fluid pumped slowly through the blood circuit in a loop. The source of fluid may be elevated, or the pumping may generate pressure, such that priming fluid is forced through the membrane of the filter and out the air vent(s). In embodiments, the vents are hydrophobic which prevent fluid from being ejected, so the priming system can run without intervention.
