I don't know what an old style with a locking screw and lever is. But if they are from 1972, they may be tapered plug sea cocks. These are robust and can be easily lapped to restore sealing and function. See Servicing Tapered Cone Seacocks Photo Gallery by Compass Marine How To at pbase.com
But sea cocks can corrode, particularly if the bonding was compromised. I had a Perko ball valve seacock that the top came off in my hand with the boat in the water. That was scary until I got hauled and replaced it.
These ball valve types can't be serviced other than maybe to squirt some silicone grease to lubricate the ball. Take a hard look for corrosion and if you are on the hard you might want to unscrew the top to see if the threads are corroded. That is what happened to mine- the threads on the top bushing were gone.