Thread: Weird brake pedal behaviour

Sounds like you have exhausted the vacuum in the booster by covering the pedal. The vacuum is released every time you take your foot off the brake.
The vacuum will only return once the pedal is completely released. As the vacuum is obtained from the inlet manifold (I assume you have a petrol engine) it will return quickest at idle (highest vacuum).
It would be better if you use the engine as a brake in this situation if you can by using a low gear and hold the brake down instead of pumping it will help preserve the vacuum in the booster.

Phil.

Cheers Phil.

Thanks for confirming my bad habits
I shall try to be a better driver in future

Phil B is a little off on how the brakes work.

Vacuum is built up in the booster whenever there is manifold vaccuum on the engine and the brake pedal is not being actively applied (holding or released) . For you to loose vacuum while you are covering the brakes means one of 3 things

1. you have out of round drums that are causing a pulse on the MC which is causing the rapid actuation of the booster this is burning off the vacuum faster than the engine is providing it (and its most noticable when doing say 45 kph in 4th and riding the brakes to hold the speed)

2. you have a leak on the balance seal.

3. you have a leak or blockage somewhere that is not permitting full vacuum to be generated within the booster except when its fully relaxed or given enough time to develop (think collapsed vac line or blocked check valve)

he is right with his reccomendation to gear down as part of your braking process.

In a vac servo brake assisted non vac pump series (IE most of them) you should always gear down as part of your braking process so that the higher engine RPMS assists you in braking by means of engine retardation but also so that the higher RPM against the closed butterfly of the carby gives you more ability to draw on vacuum to assist the booster.


Try a simple test first to asertain where the problem might be and how bad it is.

with the vehicle parked in neutral pump up the brakes untill all vac assist is lost. hold the brake pedal on with full force and then start the engine, the pedal should sink. on average it should sink about 1/2 the full stroke length of the pedal from where you get it firm.


Perfect world

With full vac assist built up the pedal should never get to below the 1/2 way point under normal braking condition and have about 2 inches clear to the floor (remove your floor mats) in full emergancy both feet on the pedal and trying to eject your self out through the rear bulkhead braking

with no vac assist you should be able to pump up a hard pedal within the top 1/4 to 1/3 of pedal travel after no more than 4 strokes of the pedal, Upon restart and the application of vacuum the pedal should sink no lower than half a stroke and must stop before it is hard on the floor or the pedal stop.

let us know how that goes for you.

if you have one it would also pay to put a vac gauge on the vac line from the engine and watch what it does as you do the tests.

Thanks for that Dave.
The difference is you are assuming their is something wrong with his brake system
I am assuming there is not.
Ozdunc is covering the brake and I am assuming he is bleeding vacuum that way.
The only reason I assume that this is the problem is from having done it myself.
Regards,

Phil.

if your brake system is not holding vacuum at a constant pedal position other than full up..

there is something wrong with your brake system..

usually

1. a leaking balance valve
2. a drum out of round.
3. a vac leak meaning your not achieving a proper vacuum or enough vacuum replenishment after application.

even a tdi300 at idle on long runs with a nominal booster holds enough vacuum for constant and consistant brake manupulation.

of course it could also be an engine problem where the engine is not producing vacuum at idle (leaking valves, poor tune, timing out, manifold leak, contaminated vac trap, excess crank case blow by (depending on how the vac lines are all hooked up), blocked vac port, collapsed vac line and incorrect placement of the vac fitting)

Ok. With engine off it took 4 pumps to solid with my full weight on it.
Turned on engine and the pedal sank minimally.

With full vac assist I pushed down as hard as I could and I couldn't get the pedal nearer than 2" to the floor.

Usually in normal use the brakes are great, I can lock them up if necessary and they've good modulation.
I would have thought if my drums were out of round I'd have felt some pulsing at the pedal when braking?

Originally Posted by Ozdunc

Ok. With engine off it took 4 pumps to solid with my full weight on it.
Turned on engine and the pedal sank minimally.

With full vac assist I pushed down as hard as I could and I couldn't get the pedal nearer than 2" to the floor.

Usually in normal use the brakes are great, I can lock them up if necessary and they've good modulation.
I would have thought if my drums were out of round I'd have felt some pulsing at the pedal when braking?

if its only slight not always, theres a bit of free play between the pushrod that hits the MC and the bit thats on the pedal, thats where the balancing valve bit does its thing.

to check for it disconnect and block the brake vac line then try the brakes with no boost assist while your going at about 15-20 kph if you feel any slight pulsing then you've found your culprit.





ok so given the general layout from the top cutaway, see the "air valve" and how its between the push rod from the brakes and the push rod to the MC and theres a rubber disc in there on the MC side which is the reaction disc?

then in the second diagram (and yes its a dual chamber unit but ignore that bit...) there is a valve return spring, valve plunger and poppet.

both diagrams show the booster "at rest" and in theory by balancing the 3 valve components I mentioned above the diaphragm plate will remain stationary at any poistion you want to put the pedal at.

now IF you have a slight amount of out of round the MC will pulse backwards and forwards, if the amount of this pulsation is less than the motion required for the physical gap between the 2 push rods to close up the diaphram plate will move backwards and forwards a fraction of a mm but every time it moves it will be burning up vacuum by the action of the valve.

More likely..

either you have a low vacuum supply along with a leak on the the valve poppet/valve plunger seal OR your valve poppet is leaking when in the other than fully released position.

Originally Posted by Blknight.aus

......

either you have a low vacuum supply along with a leak on the the valve poppet/valve plunger seal OR your valve poppet is leaking when in the other than fully released position.

OR he is jiggling his foot up and down while braking. (Or some combination of "all of the above")

John