[jor]

Jim,

You are certainly right to worry about coolant, as if something goes wrong the head is finished..
As no doubt you have seen from many posts the symptoms you describe are from the joints in the pcb, diagnosed in my car by thumping the dash, which results in a temporary cure. Although several posts recommend the solder method, none actually point to which are the joints to solder - should anybody know please say!
If you cannot get the gauge to work, all you can do is try to minimize the possibility of coolant loss. If the hoses have never been changed perhaps now is the time if you can find them (I think that volvo have stopped making them) - ebay does have 350655861205, but that's for the 760 and I don't know whether or not the measurements are the same.
In which case all that's left is the admittedly expensive method of using evans waterless coolant, which boils at 375 degrees, so the pressure in your system won't exceed atmospheric. Besides the cost of the stuff itself you are supposed to ensure that all the water is removed by using some kind of flush.
Another precaution taken by some was to get an 850 coolant reservoir, which has a coolant level sender, so if you know somebody who knows how to connect it you can be warned of loss.
BTW I could not find the brake pad tool so do not even know if it is volvo or not...

jor