Due to lack of exposure to them, old Central valves seem to be one of the top valves that intimidate new journeypersons when they come across them in the wild. I mean, I get it. They’re massive. They never have paperwork.* They’re blue. But at their core, old Central valves are just differential dry valves: Air pressure above the clapper is the only thing holding back the water under the clapper. On the surface, they are about as straight forward as dry valves get. With that being said, I have never met one that wasn’t temperamental and completely unforgiving. When you work on these, you need to be thoroughly and take your time up front, or it will absolutely pop on your repeatedly.
Now I know your first instinct is going to be to reset the clapper, but here is where a little patience is going to pay off huge in the long run. You need to clean the inside of this valve like your drive home depends on it. I’m talking rust. I’m talking corrosion. I’m talking barnacles. I’m talking anything from sludge, to releasing compound, to oatmeal. you need to find water, rags, and brushes and scrub everything or this won’t set up.
Now I got lucky with this valve and the clapper has rubber for seals around the intermediate chamber. There are Central Sprinkler Company valves out there that are brass on brass with no rubber present, straight from the factory. These valves set up exactly the same, but something to keep in mind is to be extremely careful cleaning these valves. Never use anything abrasive like a file, grinder, or sand paper to clean up the inside of a dry valve. If you start taking away material from the brass seat inside the valve, the clapper will never set properly again.. And that’s expensive.