"but the reservoir was low at the MIN level"

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There might very well be a leak in the system, somewhere. That same reservoir feeds both the brake and clutch hydraulics, so a low fluid level might also have to do with worn brake pads. As Frank said, if the fluid were leaking at the slave cylinder, then you'd probably see fluid leaking onto the ground near the transmission. But a master cylinder leak might not be so clearly visible: it might be dripping down the firewall, between the firewall and the carpet padding, where you would never see it without pulling back the carpet and looking for it.

If topping off the fluid restored the pedal pressure, I would say there's a good chance that there is a leak in the system, somewhere. If so, make a point of replacing both the master and slave cylinders at the same time. In my experience, once one goes, the other is not far behind.
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Jim Rothe, '99 S70 T5M,
http://www.jimrothe.com/volvo/index.html