Volvo seems to have one additional peculiarity that I haven't found on any other clutch hydraulic system that I've ever worked on or heard about: if you use a pressure bleeder at anything less than about 20 psi -- or a vacuum bleeder at any pressure -- the master cylinder allows air into the lines. It's as though there's a spring-loaded air bleed valve within the master cylinder that's stays open at any lesser pressure, forcing more air into the system than fluid, and resulting in the collapsed pedal that gave me all the trouble that I described earlier. It was very strange, but the cause and the solution are both repeatable: use low-pressure or vacuum and you'll get no clutch pedal; use higher pressure, and you'll have a pedal. Even after flushing at higher pressure, the pedal is never where it should be (or as firm as it should be) until after about a block of driving, but at least I've found the secret to bleeding this thing. I hope this tip saves you some time!
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