I changed my coolant yesterday, and I followed the pictures here to locate the two petcocks (one lower left front under radiator, one rear right back of engine):
http://www.volvospeed.com/Repair/thermect.htm
The only thing that surprised me was the amount of coolant I was able to get out of the back petcock. For all the effort of getting under there, I think I got about 12oz of fluid max. The front yielded about a gallon. I closed both and flushed with water. Repeated drain procedure and got about the same. By the way, after troubling over ingredients, additives, reports of no trouble with DEXCOOL, reports of trouble with DEXCOOL, etc I decided to go with OEM fluid, and it costs around $20 today (posts of a few years ago showed around $15). The cost doesn't seem too bad if you consider you do it every two years of so.

Anyone else have my experience with volume of fluid; did I go wrong somewhere? TIA, Don '95 854T

12 oz is 1.5 cups - I think I've gotten more than that. Might help to squeeze lower rad hose or put a bit of air pressure in the system.

But maybe nothing is wrong - When uncertain about the fluid I have measured the amount I had to add in order to refill the system. With the 850 I filled it and it took less than the specified amount. Squeezing lower hose helped get more in. After a day or so (warming up engine, driving, and letting it cool thoroughly), an air pocket evidently floated up and I was able to add the rest. Since the correct amount went in, I decided everything was ok. This has happened twice over the years.

Alternatively you could drain the system (collecting coolant for reuse) and measure the volume then.