I just purchased a 1976 Mercury 20HP and the motor seems to run great. Both cylinders have around 90psi compression (on pull start) and runs good. However the lower unit has a leak. I took out the drag plug and air screw and nothing came out. I put lube in it and it leaks out around the prop. I pumped lower unit with air and i can hear the air leaking from behind the prop. What seals are back there? Do I have to drop the whole lower unit? Do I need any special tools for the job? Any info on this would be great! It is a 1976 Merc Model 200 20hp. I ordered a repair manual today should be here next week.