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With vehicles fitted with ABS it is advisable to open the bleed nipple when retracting the piston plus removing the cap from the master cylinder. Forcing fluid the wrong way through the ABS control valves can cause them problems.
Is there a constant hiss from the footwell area or only when the brake is depressed.
With the engine switched off and the servo emptied you could try bleeding the brakes to see if it helps. I had a similar problem a few years back on a Honda - thankfully bleeding the brakes cured the problem.
Currently in madnoel10's garage:
Honda Civic 1.4l