AP 50 cruise control - how to use it?

G'day forumites, I am sure somebody here had installed the above cruise control and got it working successfully.
I have just completed installing one in my old troopie. It passed all the diagnostic checks, I even checked the vaccum from the pump which is -0.9 bar. Its a diesel and has a separate pump for the vacuum.

However, the thing does not do what I think it should, I should mention here that I have never used a cruise control before so the following may be silly questios;

On the command module, first I press the on switch, red LED tells me power is on.
I then accelerate to my chosen speed (80kmh) and push the set button.
What should happen next? The car does not hold speed when I take the foot off the pedal.
What is the RES button for? no mention of it in the road test instructions, it only says to check it - check what??

The manual is not too helpful with the road test issue.

I have fitted two magnets to the tailshaft flange and get a good output from the speed sensor (test LED is flashing nicely). I have not cut the black wire loop yet, would that do the trick?
I have tried the 3 settings of the sensitivity switch - makes no difference.
Next on the list is taking the vacuum actuator apart unless somebody has a better idea.

thanks in anticipation,
Klaus