Sinking brake pedal

Have a problem with brakes on a year 2000 S40 1.8 The brake pedal sinks accasionaly and never feels hard. Have renewed the master cylinder and this made no difference. Brake pedal feels firm when engine not running so I don't think it's hydraulics. Does this sound like a faulty servo? Appreciate any help

are you losing fluid? keep an eye on the resovoir (sp). maybe your brakes could simply have air in the system! regards cavey

A servo assisted brake pedal will always fall to the floor when engine running if you keep your foot on it for long enough, the fact that it is firm with the engine off would tell me that you probably have no leaks or air in the system. When you are driving the car, do the brakes work ok? After turning off the engine do you hear a hiss when you operate the brakes for the first couple of pumps? this would suggest that the servo is holding pressure. If not, then a check of the servo vacuum hose and connections would be a good starting point. What rear brakes has your vehicle got? If they are drums and shoes, this could be an indication that the shoes are wearing or badly adjusted, or possible leakage from the rear cylinders. I am assuming that you have checked the front pads and fluid level and there are no leaks present in the system.

Latest diagnosis 

It's now obvious that the brake pedal actually sinks when the engine is not running and the servo inoperative. A few pumps and the pedal is solid but after a minute or so it starts to sink again. Have bled out system once more to no avail. Took for a test and gave the abs a good workout, two wheels on grass and two on tarmac thinking a stop valve might have stuck, no difference and abs worked ok. Any experts out there???