Brake pedal fade drop?

Have a 92 Passat had new front pads and rotors installed few months back. When applied the brakes at a high speed first time the pedal seems to not respond then when you push down fully the brakes start to work. After the intial pedal fade the brakes respond well with no problem. But when brakes are needed quickly at high speeds you feel the pedal is not responding quickly enough. First i thought the pads are worn but they are fine. The rear brakes were inspected at the time of the front service and are okay. My gut feeling is air bubbles in the brake fluid (but how)or master cylinder is gone. The problem is brakes feel good most of the time making you feel everything is fine, then you gently touch the brakes at a certain time and nothing happens until you slam your foot down all the way luckly nobody has been run down, lots of sweat coming down my brow. Anyone suggestions thanks.

How old is the fluid?

How old is the fluid?

Since 1992 when i bought the car. I don't know if the fluid is bad but the problem happens when the car is cold or hot. So if the fluid at higher temperatures could give way but that doesn't happen.

It's not hard or expensive to change, I'd do that first.

My master cylinder went kaput a year ago. It felt like the pedal would start braking at farther back position every time I stepped on it. Get that replaced immediately with a NEW unit.