While my cruise control master switch light was working, my cruise control wasn’t. I replaced the 3 vacuum lines which were cracked/broken. I still did not have a working cruise control. I asked my mechanic to look at it and he checked everything; pump, vacuum integrity, switches, electrical, wheel speed sensor. Nothing was broken. We took it for a ride and sure enough it worked.
He told me two possibilities; I wasn’t holding the set switch long enough to get the vacuum in the lines high enough or when I reinstalled the Brake Vent valve, I didn’t pull the plunger out to full (which I didn’t). He said the plunger was self-adjusting and he always pulls the plunger before he re-installs it. I didn’t know about the plunger so it might have been stuck in the vent position when I installed it.

1996 Discovery SE7, automatic, 119000 miles, new fuel pump and hoses, 60k service mostly complete.