1) No
2) Only slave cylinder on a Volvo 2/7/9 series is is if you have a hydraulic clutch actuator...in that case you have TWO master cylinders. One for the clutch and one for the brakes
3) Most likely it is a failing brake master cylinder master cylinder

A thirty year old master cylinder does not owe you anything. It is likely that it is corroded on the cylinder walls outside the normal sweep of the piston. This is especial true if you and the PO drove gingerly and politely to church on Sunday and never had to slam on the brakes.

Then one day...you do have to panic stop (or you bleed your brakes with the pump, pump, hold method and let the pedal sink further toward the floor that it normally travels). In very short order you will begin to notice the sinking pedal.

Moral of the story is, sometimes you should just drive the snot out of your car.