Sounds like your aux circ pump is dying -- it shouldn't leak even at 20 psi hot. A little residual pressure is normal, and a small leak will make it works -- air will get drawn in at the small leak rather than via the radiator cap.
I'm suddenly loosing coolant in the Volvo, which leads to poor milage and drivability problems when the cold start device stays on (no injection timing increase with engine speed). I smelled hot coolant last night, so I think it is time for all now hoses and a change to fresh coolant, along with new glowplugs and a check on the d.....d glow plug relay -- I think the glowplugs aren't heating proplerly.
Mysterious coolant loss, especially if accompanied by high coolant pressure cold or oil in the coolant tank is a sign of cracked head or blown head gasket. A blown head gasket will eventually allow enough coolant loss to overheat the head, at which point it will crack. Best to replace gasket before more damage occurs, especially hydrolock!
And anyone else faced with a cutoff servo replacement on a 603 -- pull windshield washer reservior, take two Cresent wrenches, and take off the ALDA (hex flats on ALDA and captive nut) -- it tips over toward the engine and rotates out from under manifold. This will give you enough clearance to remove the servo. Total job was less than two hours, next time will be on the order of 30 min. Much faster than removing the intake manifold.
Get a flex socket for the glowplugs, they can also be removed without removing the intake manifold.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!