Your mechanic is wrong. The shock absorber is for damping purposes. It won't prevent your brakes from being actuated when going in reverse. I wonder what he ws trying to adjust!
If you want to prevent your brakes from actuating going in reverse, just put in a reverse solenoid. Usually, these are only needed for disc brakes but some drum brakes have similar problems.
http://shop.easternmarine.com/index.cfm?fuseaction=catalog.prodInfo&productID=4444&categoryID=0
You can also disable the surge unit for your problem by drilling into the coupler and inserting a bolt or pin to prevent the master cylinder from being applied.