I've been having a regular problem where, usually right when i start my car after it's been sitting for a while, i'll back out of my parking space, hit the brakes to stop (to shift into drive), and while i'm braking, i hear a 'sproing' sound coming from the area of the brake pedal or master cylinder, and i lose brake pressure. It feels like i only lose pressure to part of the brake system, but if i release the pedal quickly and re-apply the brakes, it's usually ok... sometimes it happens a couple of times in a row. This problem occasionally manifests when i'm driving slowly, but at most times, the braking power seems very solid. The problem hasn't manifested a single time when i have been going fast and applied the brakes, but that could be a coincidence.

I spoke with a mechanic friend, and he thinks it is either air in the system or a failing master cylinder. I don't think it's air, since the pedal is quite firm... just manifests these sudden brake power losses, usually right after or shortly after i've started the car and begun to drive. I did bleed each of the calipers a little bit today, and didn't detect any air bubbles. The ABS light does come on, randomly it seems, about 1 out of 3 times i start up, and either remains lit if it started lit, or remains unlit if it started unlit. There are no leaks (the master cylinder area is spotless). The fluid is clean and good.

The vehicle is a 940 Turbo. I was planning to replace the master cylinder tomorrow, and i'm hoping to get some input from the experts before i do so, to avoid wasting time and money if there is some other likely cause.


edit: I'd like to add that when i do lose pressure and hear the sproing sound, it doesn't feel like i lose all the pressure... feels a bit like i'm losing pressure to only some of the calipers and that the pressure has become imbalanced.