I got a 1972 914 1.7L about 3 months ago, and have been working on getting it on the road since then. So far I've replaced the fuel lines throughout, and the front flex brake lines (the passenger side was leaking). After replacing the flex lines I've bled the whole system twice (so I got out all the cruddy looking brake fluid).

My problem is that every once in a while, maybe once in 3-4 drives, I would press the brake pedal and it would be much softer than usual. It already is not very hard to begin with, but this is soft enough to get my heart racing (haven't tested to see if it goes to the floor, but it might). After pumping it 1-2 times, the brakes feel normal again, and stay this way for the next few days. (So it's not happening every time.) When I showed it to my mechanic he stepped on the brake pedal and stayed with his foot on it for a while, to see if it slowly loses pressure. He said it feels normal. But he also said that if it has gone soft on me a few times he would probably try to replace the master cylinder preventively.

Is a faulty M/C the most likely culprit? Or should I try to bleed the brakes again? I have no leaks any more now, and the brake fluid level stays perfectly constant in the reservoir.

As an aside, the caliper on the driver side front must not be retracting fully, because that wheel always feels a bit hotter than the other one after driving. Could it be that I am boiling some brake fluid and that causes the pedal to go soft? (I would think that if that happened the pedal would stay soft for a long time, wouldn't it? So it would not go back to normal with 1-2 pumps.)

If you don’t know the age of the rear flex lines change them as well. Not your problem but 30 bucks for the pair is cheap compared to the risk driving with old ones. Your problem sounds like your calipers need to be replaced or rebuilt.

and if all that stuff is old, I would suspect the MC is old as well. 914rubber carries them at very reasonable prices and you'll never regret having your brakes 100%.

I suspect the master cylinder as it sounds like an internal failure. Surprised it's not more consistent.

I sent my calipers to PMB and didn't look back. They're as good as people say.
http://www.pmbperformance.com/catalog.html

could be boiling the fluid, which causes a momentary loss of pressure, then when the fluid cools the pedal returns to normal

More likely you have more than just the one caliper sticking than boiling fluid. When you step on the pedal you fill the space behind the caliper piston with fluid, if they stick that fluid stays there and the next pedal push will feel different. Fix your calipers and change those rear flex lines. I agree with the previous post on the master, if in doubt change it as well. Brakes are about the most important thing you can work on.

Yes calioes and lines 1st. Then you'll have it narrowed down quite a bit.