I'm an old guy from the days before ubiquitous power brakes. When I was young a brake peddle that slowly collapses to the floor with steady foot pressure was a sure sign of a failing/failed master cylinder (fluid leaking past the seals). My '94 Camry began doing this yesterday. When I said to a friend "I guess I need a new master cylinder," he said, "It's probably the brake booster." I guess that's possible, so . . .
Could it be either of these?
If so, how can I isolate which of the two it is?