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If the stopping power of your 2CV responds to rapid pedal force application but not to gentle pedal force application. . . go on reading

Symptom: When, in order to bring the 2CV down below sonic speed, the brake pedal is applied gently, the pedal continues to drop towards the floor until break pedal hits the floor

Cause: 2CV Master Brake Pumps have a tendency to  "bleed back" fluid to the reservoir when gentle pedal pressure is applied.  The symptom carries some resemblance to air in the braking system. The poppet valve arrangement inside the pistons failing to close properly is causing the bleedback problem.

This description applies to the standard dual circuit front disc brake & rear drum LHM system fitted to 2CV from 1980 and onwards.



The poppet valves are marked * for front and ** for rear. Particularly following longer periods of not being used, varnish builds up at the popett valve seat and the leakback causes the pedal to sag when pressed gently.

There are repair kits available but those only contain the four piston seals and the two reservoir seals. In order to access  poppets, remove the pistons from the cylinder and blow into each piston from the front end. If air bypasses the popett you will certainly notice it. If leak is observed, remove the poppet valve(s) to be cleaned.

The mesh at the front of the poppet valve and the spring/popett must be removed and carefully inspected for damage. If there is no damage, the seat area can be cleaned carefully with a Qtip or similar.  Only use LHM as a cleaning agent. Re-install.
Make sure that the mesh locks in neatly. If not, discard the unit and go for a new one.

Remember that the poppet valves are not actually meant to be serviced and precaution must be taken. These are your brakes, remember!

Installing is the reverse process.

The procedure is only meant to serve as an emergency procedure and a new master unit should be installed at the earliest possibility.

Updated by Terje Ellefsrod 2008-04-27