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I just had the same problem on my '77 johnson 140hp. I had a broken o-ring on the shuttle valve.
Take a look at the attached diagram. The red circle in the diagram shows the check valve and shuttle. The check valve is actually the large slotted screw (see attached pic) and the shuttle is located directly behind it.
To remove the shuttle valve, raise the motor all the way up and remove the check valve. Once you do that you will see the shuttle. You should be able to reach in there with needle nose pliers and pull it out. There is an o-ring on the shuttle that may be broken. I bought a new o-ring at autozone.
***Important note: There is a second check valve behind the shuttle (blue circle in attached diagram). You may have a pressure lock on this valve.
To release the pressure: Remove the check valve and the shuttle. You'll see the reverse side of another check valve. With a small punch of the proper size, insert the punch into the small hole of that check valve. Now, carefully tap the punch with a hammer. This will force the check valve to release the pressure and equalize as it should be.
Hope this does it for you!
