In at least some implementations, a locking plate may be received between the cover plate and the adjustable valve and be movable to a first position permitting access to the adjustable valve and a second position inhibiting or preventing access to the adjustable valve. In some implementations, a latch assembly may releasably retain the locking plate in its second position. The latch assembly may include a finger carried by the locking plate and releasably engageable with a stop when the locking plate is in its second position.