Thanks for making more aware of that possibility, Bruce. Turns out you were absolutely on the right track. The master cylinder connection at the booster and inside the bottom of the booster seemed dry. I've now pulled every caliper to carefully checked for fluid behind the boots. During my heavy footed panic stop I managed to push a load of fluid past the seal of one front brake. There was a fairly large amount of fluid suddenly unaccounted for, so when I originally inspected the calipers and everything was dry I just assumed it was more likely the worn master cylinder dumping it back into the booster. My 900 uses a single piston (clamshell style) front brake and the dust boot is very well sealed to the piston. Unless the piston is fully retracted a fairly large volume can hide behind the boot. I drained and cleaned the piston and yesterday put in a new master cylinder carefully attempting to do it without a full bleed as I had just pressure bled the entire system not that long ago. I'm still not happy with the brake feel so I'm going to re-bleed and if that doesn't satisfy me then the booster goes back on my suspect list as I can still hear occasional hissing when I take my foot off the pedal. My wife also has a 940, higher mileage like mine and with an identical brake system. The brakes are as hard as a rock and I won't be satisfied until mine is as good.
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Dave -not to be confused with a real expert

Thanks, I'll certainly verify the booster leak as you suggest before making the final diagnosis. I came to the booster theory reluctantly and only by process of elimination knowing that the master cylinder was already due for replacement and I'd externally inspected fairly carefully around the master cylinder, ABS unit, brake line connections, hoses and calipers. I'm mostly interested in what my options are and how high on my priority list this project needs to be if my fears are correct.
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Dave