Used to top up the Hydrive hydraulic fluid in a system by pouring it into the top of the helm unit.
Suits: 101, 102, 401, 402, 500, and 501 Hydrive helm units. (These are usually found on the Hydrive Admiral and Commander range of kits). It does not suit the helm HD Series (specialized heavy-duty series).
Always use Hydrive hydraulic fluid as some hydraulic fluids can damage seals. Do not overfill as there needs to be room for expansion.