How do you know your A/C compressor is gone? If it indeed is gone, you can pick up a compressor/orifice valve/accumulator for cheap on ebay ($120). I've had that same compressor in my car for three years and its running strong.

As for your cruise control problem, start with making sure all your vacuum lines are good. Then also check the yellow wire to the speedo and make sure the solder hasn't started to crack. Finally (or firstly, depending on some people) check your brake switch to ensure its properly adjusted and the vacuum line to the switch itself is okay.

 

Re the Bulb Relay: The lesson more and more is that when somebody says "I changed such and such..." and then goes on, it doesn't mean that the replaced item is for sure eliminated as the culprit...

Re the Cruise, the switch to check is the Vacuum Dump Switch at the brake pedal (different from the brake light switch). It is often maladjusted.

 

My problem was almost identical. My 1990 780 cruise control went out about three months ago, and last week some crazy guy behind me honked and raved when I slowed for a right-hand turn. I didn't think about that until I decided this week to work on the cruise control. I pulled the cruise control vacuum/electrical switch from the brake pedal arm and wondered whether Volvo used the same switch for cruise and brake lights (I once had a 1979 Ford that had a dual purpose electrical switch). No; it's a separate switch and much harder to get to. I tested the brake lights--all remained dark. I tested the operation of the brake switch in-situ with my multi-meter and it showed open circuit with no change when the pedal was pushed, but it's a bear to get probes, hands, and line-of-sight to the switch contacts all at once and then push the pedal. I did have voltage (12V) at the yellow-green wire to the switch.

So I ordered a new brake switch, wondering how the hell I was going to get the old one out. It's buried above the cruise control and I have no idea what size nut is on the back--I hoped it was the same as the nut on the replacement, which would at least give me a fighting chance of using the right tool.

While waiting for the switch ($40+ and two days wait from Volvo, $17 and a day wait from local parts store, and (my choice) $11 Beck Arnley from Amazon Prime) I decided to look again at the forums (this one, since 780's aren't too common but share much of the running gear and electrics with 740's.)

This morning, after going through this thread, I decided it was time to change out (yes, nothing new here!) my bulb failure relay with one of the two spares I had picked up at Pick-n-Pull. Lo and behold, the brake lights now work! On reflection, my in-situ switch test would have been easier and more certain had I connected the hot yellow-green wire to the switch and simply looked for voltage at the other terminal when the pedal was pushed.

So my new $11 brake light switch is arriving today and will go into the parts box. And my cruise control works again too, so I'm one happy camper!