The brakes on my 1979 240D have developed a problem. It's an intermittent problem as I can't make it happen every time. When it does happen it will happen like this. When applying the brake, the car will slow down like normal, but just before comming to a stop, the "feel" of the pedal gets real hard and the braking power seems to disappear. doesn't happen every time though...maybe 1 time out of 4 or 5. Also, at a stop, when pressing the brake pedal hard, it seems as if the pedal will very slowly, almost imperceptably, go toward the floor.
Is this the signs of a leaky caliper piston, or master cylinder? Or something else?
Thanks in advance for any insight.