LGB boiler pulling vacumm

the vacuum part is normal for all Steam systems. When the fire is gone the steam goes away replaced by a tiny amount of liquid water so a vacuum must result.

The pressuretrol should be set as low as possible. If you have a vapor system it should be even lower.

A pressuretrol doesn’t feed pressure, it just turns off the fire

The odds are pretty good that the pigtail from the boiler to the pressuretrol is clogged. I'd take it off and check the opening into the boiler (must be free and clear) and then I'd replace it with one of brass. Check the opening into the pressuretrol itself as well.

The fact that the system pulls a noticeable vacuum — while somewhat normal, as @ethicalpaul said, suggests to me rather strongly that the poor old Hoffman #75 main vent is shot. Operating under high pressure like that they don't last forever. I'd recommend replacing it with a Gorton #2. The odd vent on the radiator probably doesn't belong, but it may — or may not — need to be removed. We'll see.

Now. I wonder what type of vapour system we have here? Can you post a picture or two of the inlet valves on the radiators and of the outlet elbows? If possible clean them up enough so that any markings or tradenames are visible?

Also, are there any odd devices or hardware around where the dry return(s) finally drop to the wet return and the boiler? If you could manage a couple of photos which clearly show the piping arrangements of the steam header and near boiler piping, and of the return piping, that would help too.

Are you quite sure that Hoffman was a 75 and not a 76? They look a lot alike, except the number — but the 76 seals on vacuum and, barring leaks anywhere else, will hold a pretty deep vacuum for quite a long time.

As @ethicalpaul said — harmless. In fact, in a good tight system if you've got the cash I'd often suggest a 76 — but oh dear they are pricey… and the ROI just isn't there with a modern small boiler.

Looks like the typical mishmash. The one valve where you showed the top plate with the "L" on it was probably made by Lunkenheimer.

You have two ways to go. You can either get orifice plates from Tunstall and install them in the valve unions:

https://tunstall-inc.com/macon-controls/steam-radiator-inlet-orifice-plates/

or you can replace all the valves with orificing types from MEPCO:

https://www.mepcollc.com/product/swrf-b-regulator/

or

https://www.mepcollc.com/product/swrf-c-regulator/

The radiators which still have the old valves can be left that way, assuming that the valves still more — which, with a little encouragement, they probably do. Just replace the ones with the regular valves.

Balancing one of these systems is really simple — since the regulating valves can be partially open, unlike one pipe. just adjust the valve on each radiator until you get what you want in that space. Again, unlike one pipe, there is very little interaction between radiators.

But I'll mention it again — don't even try until you get the pressure under control!

Yes keep the existing pressure control — I, at least, like to have two on anything much bigger than a teapot. One as a control device, and one as a safety. And 5 psig is fine.

I pick the Gorton #2 as it has much more capacity for venting than the Hoffman. I doubt that you'll need more than just the one.

But i do have to ask since we've gotten this far: are there crossover traps between the steam mains and the dry returns?

There has to be some way for air to get out of the mains. Usually that would have been a crossover trap — but those may have been removed (what's that thing for? Don't need it. Get rid of it!) at some time. The radiators will vent the mains into the dry returns, but things will be quicker if there are crossovers. Until we figure this out, though, don't add any more main vents than the one you have.