An improved locking assembly is provided for releasably locking a valve unit incorporated into a sprinkler housing or the like, wherein the locking assembly prevents the valve unit from being driven from the housing as a projectile by water under pressure at the upstream side of the valve unit. The valve unit includes a mounting ring normally held seated upon a shoulder seat within the base of the sprikler housing by the locking assembly including a stacked pair of snap rings installed into an enlarged housing lock groove thereby positioning a movable valve member in association with a valve seat to control water flow into the sprinkler housing. The valve unit is removable from the housing, for example, for maintenance purposes by removing the two snap rings one at a time. In the event the first snap ring is removed without prior relieving of water pressure at the upstream side of the valve unit, the water pressure lifts the valve unit from the valve seat to permit water flow into the housing while the second snap ring remains locked within the lock groove to block valve unit removal. When the water flow is turned off, the valve unit is removable quickly and easily by removal of the second snap ring.