BMW 316i Compact brake problem

Hello I don't know if this is the right forum to ask for help. Please pardon me if it is the wrong place.
I just bought a 1997 BMW compact 316i. The car is very good, however
whenever I brake, especially from a higher speed, there is a virbration. Hence I got the wheels balanced and aligned and also got the rear tyres changed. I also got the front brakes checked and they were OK.
Also the brakes seems mushy and there is a hiss whenever I press the brake as though there is air somewhere. Could this be the cause of the vibration?
I would appreciate any clues in the matter.
Thanks a lot
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sounds to me like your brake discs ( rotors for you american boys) are warped. make sure you clean up the hubs before mounting the discs. as dirt, corrosion etc here will not allow the disc to sit perfectly square, always a good idea to fit new pads when fitting new discs too regardless of condition. the hiss comes from the brake servo, it is the result of air vacuum from the engine intake manifold assisting the application of the brakes by forcing the large diaphram built inside the brake servo unit outwards to help apply the brakes. this is why the brakes are very hard to apply when the engine is not running. A small short hiss is normal when initially applying the brakes, continual hissing is likely the result of a leak somewhere in the system, probably from the servo unit itself. ( not repairable ) this noise will not be related to, or connected to your brake judder. to test the servo: with the engine off, dispell any air in the brake servo by pumping the brake pedal about 10 times. then, with your foot applying continuous pressure on the brake pedal start the engine and feel for further movement downwards on the pedal as the vacuum in the servo unit is built up. no movement here will suggest the brake servo unit is not functioning correctly. (the pedal should travel an extra inch or so) before condemning the servo unit be sure to check the one-way valve fitted along the servo vacuum pipe. Regards. Steve.

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Thanks a lot for your answer. Regarding the hiss, I tried the procedure. When I press the brake with the engine off, the brake becomes hard the hiss disappears. Then when the car is started, the brakes do go down. Does this mean that the servo unit is ok and the problem is something else?
Thanks again for your detailed answer.
steve wrote:

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Could be:
Worn control arm bushings (VERY likely). Worn tie rods Warped discs.
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