The Traction Avant is regularly parked at our place for long periods of time.
Each time, the level of my brake fluid when I picked up the car was at minimum, while I had put the car away with maximum level.
The reason turned out to be that on almost all parts where the original meager mineral brake fluid could flow under gravity along a brake cup, it actually did.
The result was a lot of filth and an empty reservoir.
The residual pressure that keeps the rubbers sealing nicely, drops off after a certain time and then this above mentioned problem arises.
The solution turned out to be upgrading to DOT3 brake fluid. DOT 3 has somewhat lubricating properties which in my experience keeps the seal between cylinder walls and brake cups closed.
I’ve been using it this way since 2015 and haven’t lost a drop of DOT3 since.
An additional advantage is that because of the lubricating effect the brake pistons no longer get stuck as a result of the long downtime during the winter, in combination with the hygroscopic effect of the old brake fluid.
It was quite a job to get the old fluid out completely, flushing with methylated spirits, blowing crosswise until all the methylated spirits were removed at each end point.
Then fill up with DOT3 and do some serious bleeding.
DOT 4 is not a good idea in any case, at least not with the original rubbers.
The additives in DOT4 cause the original rubbers to swell. If you want to use DOT4 or any other DOT version than DOT3, replace all rubbers and cups of the wheel brake and master cylinder with after market (so do NOT use old stock) rubbers and cups.
Update 3-2021: Everything is still fine with the brakes, I did remove the front brake cylinders as a precaution, cleaned them completely, flushed them out and filled them again with new DOT3.