In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
Old Cast Iron Radiators; Shut off Valve Question
Hi. I'm a homeowner not a pro but will share my experiences. I have a hot water system probably similar to yours. First, although I have not done it, breaking a valve could indeed be very bad news: Best case you will have to replace the valve at a future date. Worst case you will have several hundred gallons of water in your house. But to get to the point...
Yes, old rad valves often are worn out and will not close all the way. However if they will still turn they will often modulate the flow enough to balance temperatures. Now the trick - getting them to turn. There are several different kinds of valves. Some can be serviced without removing them and some cannot. The ones that have a cap with a (usually) square nut just under the stem packing nut can often be freed up by removing the handle, packing nut and packing, cap (by turning the square nut), then the internal valve parts, which will almost certainly require soaking in liquid wrench prior to removal. Clean everything and put it back together with a little automotive formed-in-place-gasket for the cap and new stem packing. I like to use a little plumbers heat proof grease on the valve parts but this may not be a good idea on a steam system where oil or grease in the boiler water can cause problems in steaming (I think).
Valves with no service cap must be removed from the radiator and the pipe to fix them, and if you're going to remove the valve from the pipes, I would replace the whole valve with a new one. They aren't that expensive compared to the hassle of getting one out and back in without leaks. If a valve does not have a service cap I personally consider it "pipe" and not a valve at all. It cannot reasonably be serviced, in my opinion.
By the way, did I mention that you must drain the system before you take anything apart? And know how to refill to the correct pressure and bleed the rads? Dan Holohans books are a great way to gain a decent understanding of how all this works.
My suggestion? Work on the valves in the summer, if you must.
Good luck, hope you get some responses from the pros.