I have a 1970 250C that had the same problem. It turned out to be the o-ring on the top round nut. You remove the acorn nut and lock nut. Accurately measure the length of the threaded shaft that is sticking out of the round nut. Then use a pin spanner and small wrench to back the round nut out of the steering box. You will find the o-ring on the side of the round nut. I replaced mine without to much trouble and it hasn't leaked since.