The Traction Avant is not often used.
Each time the level of my brake fluid was at minimum when I picked up the car, while I had put the car away with maximum level.
The reason appeared to be that on almost all parts where the original lean mineral brake fluid could flow through a brake cup under gravity pressure, this actually happened. The result was a lot of dirt on the floor and an empty reservoir.
The residual pressure that keeps the rubbers neatly closed, falls away after a certain time and then this problem arises.
The solution was to upgrade to DOT3 brake fluid. DOT 3 has some lubricating properties which in my experience keeps the seal between cylinder walls and brake cups closed.
I’ll be using it this way from 2015 and for the past 4 years I haven’t lost a drop of DOT3.
An additional advantage is that because of the lubricating effect the brake pistons don’t get stuck anymore due to the long standstill in winter.
It was quite a job to get the old fluid out completely, flush it with methylated spirit, blow it through crosswise until all methylated spirit has been removed at each endpoint. Then fill with DOT3 and seriously bleed.
DOT 4 is certainly not a good idea, at least not with the original rubbers. The additions in DOT4 cause the original rubbers to swell.
If you want to use DOT4, replace all rubbers and cups of the wheel brake and master cylinder with after market (NO old stock use) rubbers and cups.