Yes, I did remove the valve cover, and it was relatively easy.
There are a series of screwed-on clamps that hold the brake line, and one of them is right behind the valve cover. After the cover is removed, then one needs a phillips head screwdriver about one inch long to get the screw out. I used a phillips bit to an impact driver set and held it in small vice grips. It probably took 35 minutes to work that one screw out, and if it had been a hex-head screw, it would have taken two minutes. It would still be there if it had been tightened like the rest of them.
Of course, one cannot get to this point until he has completely disconnected the accelerator linkage that is attached to the firewall (since the screws that secure the brake line are underneath this linkage), and the brake booster and the coolant tank. When I removed the four screws that secure the accelerator linkage to the firewall, one of the nuts that snap into the firewall fell back behind the firewall. Guess where? Into the heater box.
I's starting to think I may be a Buick guy after all.
Joe