The present invention is directed to a portable apparatus for dispensing a liquid under pressure at a substantially constant flow rate over a period of time. The apparatus includes an inflatable, internal bladder constructed of a compliant, elastomeric material and an external housing surrounding the internal bladder. Further, the external housing is constructed of a non-compliant, elastomeric material. Thus, during operation, the external housing shapes the internal bladder as the internal bladder is being filled with a treatment fluid and collapses as the treatment fluid is emptied from the internal bladder.