Airplane:  172H
Pneumatic Stall Horn - "serviced" during annual inspection because mechanic couldn't get it to go off on the ground (it worked in the air), now doesn't work properly.  Have had it back to the mechanic several times, now comes on at a very low speed, about 50 mph with flaps (airplane is already in a stall when horn comes on).  I know about moving up the plate to increase speed at which horn comes on, but this doesn't seem to be working.  Also read from the C-172H service manual:  "Use 579.6 Presstite sealer between adjustable plate and wing leading edge to ensure postive seal."  Anyone know anything about this?  My mechanic doesn't seem to.