A seacock is a critical safety device. It should be properly maintained according to the manufacturer's instructions!
The information presented here is not a recommendation! These are my observations and methods that worked for me. I started by following the manufacturer's instructions and continued investigating until I found the sources of the problems causing my seacocks to not work the way I wanted.
My 2000 Searay 380DA now has six seacocks. These are all Apollo Conbraco
hull-mounted using the thru-hull and the flange attached to cored sections of the hull. These are attached w/ large self-tapping screws and washers.
My boat has two 1¼" valves for the engines, two 3/4" valves for the gen and the A/C, a 1½" valve for the macerator and a 1" valve I added for the bow raw water washdown.
These all worked okay. I'd lube these and actuate these but they never worked great and more lube / actuation never changed the operating characteristics.
The manufacturer was contacted to request their recommended service procedure. They suggested no lube. I pressed them for acceptable lube options and they finally stated a silicone lube is safe, but never WD-40.
The silicone lube was tried, but the lever movement remained about the same.
When the boat was out of the water I disassembled the output nipple, the drain plug and the lever mechanism.
All of the valves had one problem. Some had a second problem.
On all valves, the lever mechanism needed service. Once the lever is removed, the actuating rod and the rod retention nut is exposed. When I removed the retention nut, the two nylon washers and the actuating rod, I found crud on all three parts. Some had crud on the nut.
Those parts were cleaned of crud w/ methods that could not damage the surfaces.
At assembly, those three parts were lubricated w/ Mobil XHP222
marine grease that I purchased from McMaster-Carr
. This is a great product and is now always aboard my boat. I use this lots.
These parts were all reassembled in the same orientation. This is important!
The retention nut was snugged and not over-tightened. If the nut is too tight, then the valve will be difficult to move.
On two seacocks, an examination of the stainless ball revealed a small lip that prevented full motion travel. I used the marine grease and my fingernail to remove that lip crud from the stainless.
The ball was lubed by hand onto that sphere w/ that Mobil marine grease. I also filled the drain w/ grease.
With the handle off I performed many 360° rotations of the ball / actuation rod. I pushed grease into the drain by hand periodically.
I would never use a grease gun onto a fitting at the drain, because high-pressure grease could cut the spherical ball seal.
Now, every seacock has a very easy / smooth motion. They have the feel of a silent light switch.
It should be possible to remove the retention nut / actuation rod while floating, but I don't know.