To access the Gardena T120 diaphragm remove the oval cover at back with small flat screwdrivers to reveal the round retaining cover. The retaining cover unscrews to reveal the diaphram mechanism which can be removed and cleaned. Hopefully this solves the problem.
Dirt or sludge in here cause the timer to stay on or leak.
Stage 2 if the above fails.
Remove the front timer cover with very large flat screwdriver. Simply lever off the front timer cover. This is a brute strength operation. There is no tab or button to help remove the front. Insects and dirt can get in and stop the operation of the needle mechanism.
I do not know how to get to the mechanical timer itself. Any suggestions?