I like Groco seacocks with the rubber covered plug. Lin and Larry Pardey, Taleisin of Victoria, fitted Groco seacocks in Seraffyn and Taleisin. We fitted Groco seacocks in our aft-cabin Flicka when we rebuilt her.
In operation, the T-handle that controls the pressure plate inside the seacock body is loosened, the seacock handle is turned to the open or closed position and the T-handle tightened. Tightening the T-handle forces the pressure plate against the Buna-N rubber covering on the plug which expands the rubber and seals the seacock. If the seacocks are maintained and cared for properly, they will last almost forever.
As with most seacocks there are some do's and don't's:
1) Tightening the T-handle forces the plug at the handle end,to push against the bronze retaining ring that holds the plug inside the seacock. If the T-handle is over tightened, the retaining ring may deform and prevent the seacock from sealing. If this happens, either a new retaining ring should be installed or the old retaining ring removed and straightened.
2) Attempting to turn the seacock handle without releasing the pressure plate will damage the rubber covering on the plug.
3) Never, apply grease to the plug or inside the seacock body. Grease will cause the rubber to swell, resulting in a stuck or damaged seacock.
4) As is the case for all seacocks, including modern ball valve seacocks, the valve should be operated at least once a month. Failure to operate the valve on a regular basis, may result in a stuck seacock or damaged seacock. If the valve is not operated regularly, the rubber coating flows into the seacock's through-bore. If this happens, turning the seacock damages the rubber covering the plug. A similar things happens with modern ball-valve seacocks. The teflon seals flow into the bore-hole in the ball. When the ball is turned, the seals are damaged.
5) As mentioned before, the rubber covering the plug is made from Buna-N. If the rubber will not seal against the inside of the seacock, it may have become oval in cross-section. Simply remove the plug from the seacock, place a sheet of 80-grit sandpaper in one hand, followed by the plug and sand the plug's surface with a rotating back and forth motion. Turn the plug a quarter turn, and repeat the sanding. Repeat this operation for perhaps two turns and reinstall the plug in the seacock.
Groco, does not have any of these seacock bodies but several years ago, they had a few barrels of new plugs and parts. I just called Groco. The front desk, is just that "the front desk" and Patrick did not answer his phone. Groco (Baltimore, MD) has a website and contact information. If you e-mail Groco, mark your e-mail "ATTN: Patrick."
The person you want to talk to is Patrick. He runs the shop, been there forever and knows every product Groco every manufactured.
I will write later about seacock backing plates.