Citroen C4 Cactus Brake Booster

A rock-hard pedal is the headline — with no assistance you are pushing the master cylinder directly, and the car still stops but takes far more effort and far more distance. Alongside that expect a hiss when you press, and on a petrol car a rough or dipping idle as the leaking booster pulls unmetered air into the manifold. On an HDi, a failed vacuum pump produces exactly the same symptoms, so test the vacuum supply before condemning the booster.

It depends on the engine as much as the model. A petrol C3 uses a straightforward vacuum booster. On an HDi the vacuum pump often needs replacing at the same time, which changes the parts bill. Labour is three to four hours in either case because the master cylinder has to come off the front of the booster and the brakes then need bleeding properly. Send the VIN and we will quote parts against your exact car.

We would not. The brakes still function mechanically, so the car is not undriveable, but stopping distances can double and the pedal effort required in an emergency is more than most drivers will manage in the moment. If the car has to be moved, keep it slow, keep it short and leave twice the following distance you think you need. Towing it is the better call.

Almost always a loss of vacuum somewhere in the chain. Work from cheapest to most expensive: the hose from the booster to the manifold or pump, which perishes and splits; the one-way check valve in that hose, which is a small part that fails often; then the booster diaphragm itself; and on an HDi, the vacuum pump. Start with the hose. It costs very little and it is the answer more often than the booster is.

We source both new and good used boosters nationally, matched against your VIN. C3, C5 Aircross, Xsara and C3 Aircross units come up regularly. Boosters for the older C2 and early Xsara are usually strip-yard finds rather than shelf stock, so those take a little longer — send the chassis number and we will tell you honestly which category yours falls into before you commit.