My 5yr old 206 has developed excessive brake pedal travel. The car still brakes efficiently despite the travel which is very pronounced when the engine is running but the pedal has a 'conventional' feel when the engine is stopped. Fluid level is fine. The car is booked in for a service shortly but brake wear wasn't noted last service as being likely to need shoe and pad replacement this time. Is this likely to be simple brake wear, a problem with the power assistance or something else?