I know nothing about Volvo's....
but from the symptoms, this would be my take.
1. On the long down hill run, the radiators working and cooling the water in the radiator; and the engine is not developing heat. So, the themostats are closing and effectively blocking the pressure release path. (or severely constricting the path compared to the volume of gas being pumped.)
2. Under load, the thermostats would be open and water circulating. Thus any pressure will vent via the radiator cap. It would look like the system is pressurised. Which it is, but the pressure is caused by gas and not by the expansion of coolant under heating.
Q: does the overflow tank show signs of being flooded or had the lid popped off or something?
3. I don't think it's oil caused because the volume of the oil needed to build extra pressure would empty the sump over time. (and note, no oil was in the coolant.) The comment about the presure tracking oil pressure supports the idea of a compression leak. The pressure is tracking RPM's. Just like the oil pressure.
4. Running on an exhuast brake would have similar compressions (or less) as driving under load. (consider ignition pressures) which brings me back to point 1 and 2.
Based on the 4 points , I reckon there is pressure pathway from a chamber to the water jacket. And, the leakage would be noticeable all the time if you were looking in the right spot. But, the spots will change depending on what the thermostats are doing. (and that's why there are confusing symptoms)
Q: Have you measured the radiator pressure at the cap while under fast idle speed with the thermostats open? My bet would be that it increases to bust pressure or the cap release pressure.
Another idea would be to extend the overflow hose into some sort of loop and watch for bubbles as the radiator cap releases.
A leakdown test is probably the only way to find out which pot is shot. And, probably necessary and the least expensive method if something is cracked. btw, test all pots, even if the first one is shot.... there may be more than one gone.
One last question, again this is because I know nothing about the engine, is the exhaust manifold (or turbo??) water cooled? There may be a crack in these and it's pressurising when the brake is closed. (again, I assume we are talking about an exhaust brake and not a jake,) If they are, then, it may be possible that its being pressurised "outside" of a combustion chamber. If they are cooled, then the leakdown will show nothing and you'll need to prove 'idea' by pressuring the exhaust somehow.
jd
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