General Rear brake pad change now got hissing sound

[Daiton]

Hi all hope we all good,
I have an issue iv changed my rear pads today and now when I pumped the brake pedal I noticed a hissing sound with every pump.iv taken car out for a test drive and brake working great.
If I press brake pedal fast then you hear the hiss but apply pressure normally during driving you can barely hear hear it.
Any help would be great and thanks for the heads up in advance

 

[spampebs]

When mine did this after changing it was just the metal guard behind the disc, somehow id bent it in slightly and for whatever reason it was just touching the disc when I applied the brakes. I just bent it back a touch with a screwdriver through the alloy.

Not sure if the rear dics have this guard though.

Without hearing your noise I can't be sure it's the same problem I had.

 

[The Panda Nut]

Servo brakes do a sort of shush noise when you press the brakes. Its the vacuum activation. It should only do this once each time the pedal is pressed. Continuous hissing from under the bonnet would indicate a vacuum leak. Check the pipes from the servo to the pump mounted on the engine, Leaking vacuum system can be noisy but still allow partial servo assistance so the brakes work but less efficiently. The tip from Spampebs also sounds good. Did you change the discs too. If not check there is not a lip on the disc that needs filing.