The brake compensator valve is the most complicated part of the hydropneumatic suspension. This component is not only responsible for the operation of the brakes but forms a central part of the whole system.
Although there is a so-called repair kit for the brake valve, this only contains replacement rubber seals. These seal the valve to the outside but they have nothing to do with the functioning of the valve itself; the sealing of the working elements rely on the very close tolerance between those parts. Once the pistons inside the valve or the cylinder they travel in wear out, the valve starts leaking (this means leakage into its return lines and not to the outside world).
This is a sure sign of air in the rear brake circuit. Bleed the whole system including the brake valve.
This is common and is not a fault only a nuisance--it is because of the way the calipers sit. When in reverse, and you brake, the force on the assembly is in the opposite direction as when you drive forwards, and the small amount of play in the calipers make them clonk the other way. This also happens on Peugeots which also have the same assembly. Original brake pads behave better than cheap pattern ones.