Thinking back to my '76 F (as it is in the garage and I am in my robe), There are two things you might want to check. The first would be the obvious (and you've probably already done this) and check the lockling mechanism to see if it is tight. Check both halves, too, as either one will cause it to feel loose. The next thing to check, while you're right there, is to see if the part that is attached to the seat has been altered in any way; like having washers put underneath it to raise it up for any reason. And the last thing I can think to check is to look where the seat contacts the frame to see if there is any thing missing or out of place. I have become an expert on removing and installing my seat while I have been rebuilding my bike, but for the life of me I can't remember if there are an little rubber stoppers where the seat meets the frame or not. With anything like this, a missing stopper would case the seat to be loose.
