Also with a 1974 DSII
Is the water coming in overnight (while docked) or when you're sailing? Assuming your hull's sound, I've looked over my (non-leaking) 1974 DS II, and there only seem to be three places on my boat below the water line where water could enter the floatation area while the boat's at rest and relatively empty: the automatic bailer, the inner hull drain in the transom, and the center board through-bolt. Maybe you could dust some talc around the through-bolt and see if there's a water flow from it (use the access ports on either side of the centerboard trunk). In addition, I was so worried about the idea of leaks, that I mounted two 5" diameter clear plastic ports in the port and starboard sides of the aft well. In addition to giving me access to the bottom of the bolts holding the traveller blocks, it allows access to the inside of the inner hull drain and the transom area. Adding something like that could help you find/fix the problem if it's back there. Also, I was thinking that it gives you a way to allow water from the floatation area into the well, and then out the automatic bailer (if it's not coming in faster than the bailer can remove it - otherwise opening them up could be a really bad idea). But, the clear plastic lets you see if any water is building up. Been lucky, myself, so far. If it happens while you're moving, it could also be the holes for the control line for the centerboard, or maybe even the joint between the hull and inner hull around the centerboard. You could drain the water, and then beach the boat with the transom in the water, and see if any comes in. That could point to the bailer or drain. Or pour some water into the floatation area while it's on the trailer and lift the trailer tongue and see if any water comes out back aft.