Lost all brake pressure while sitting a second time
 

We moved my brother's W116 300SD up to our new house a couple of months ago. It had been sitting a while because work swapping the ACC to manual climate control has been slow because we haven't had the time to do much with it. Before we moved it, it had lost all brake pressure so we figured the master cylinder had gone bad. The brakes had been totally redone besides the master cylinder (new lines, discs, pads, and rebuilt calipers) so we figured that was the issue, we swapped in a known good one from a parts car, bled the brakes and drove it up. We had to move it recently and it has lost all pressure again.

Any ideas as to why this would happen? I'm thinking maybe one of the rebuilt calipers is shot but any other insight would be nice.

Similar thing happened to my TD. No pressure after sitting about 6 months. Swapped in a new MC. Sat on the street a few days (a couple of weeks later) and had similar loss of pressure. I actually ordered a replacement MC from the supplier since I figured the new MC was failing. Haven't swapped it in yet because I drove the car a few more times and it hasn't happened again yet. But it's been sitting in the storage lot for a week or so and when I go to pick it up I'll see if its happened again before swapping in the new MC.

Well it suggests a leak. you should be able to find a puddle if you lost all that. Also could be a loose fitting, you'll just have to look until you find it.

The only other thing is it could be perhaps leaking into the booster....?

I suppose there could be a bubble that has moved up to the MC but seems doubtful if you drove it some distance to get it home.