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

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.



> wrote in message
oups.com...
> 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
>

> 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.

Could be:

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

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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:
> 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.
>
>
>
> > wrote in message
> oups.com...
> > 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
> >

> wrote in message
oups.com...
> 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.

the brake will become hard with the engine off, this is normal as you wont
have any assistance from the servo, (this relies on vacuum from the engine
to operate)
the hiss will dissapear after a few pumps too as you are using up any vacuum
stored in the servo (the one-way valve in the pipe-line keeps the vacuum
stored there)
if the pedal moves further down when the engine is started then i would
suggest the servo is ok, however if the hissing is continuous then its clear
there is a leak somewhere even if the servo is operating.(it may not be
operating fully)
is the engine idle speed affected when you apply the brake pedal with the
car stood still? if so this is another indication that vacuum is being lost
somewhere via a leak.
http://www.kfz-tech.de/Engl/Bremskraftverst.htm
this is a detailed drawing of how a brake servo works ( not specifit to
BMW's)
Steve.




>
>
> steve wrote:
>> 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.
>>
>>
>>
>> > wrote in message
>> oups.com...
>> > 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
>> >
>