Weird brake pedal behaviour

[Ozdunc]

Yesterday, heading to the garden centre, we're driving in reasonably heavy traffic rolling down a slight gradient. I'm covering the brake and pushing down on it every now and then, and probably not fully releasing as the traffic was too slow for that, when the pedal went dead hard and I really had to push to get the brakes to work to come to a stop at some lights.
Lifted off the pedal and accelerated away, pedal feel has been normal since.

Its got a booster plumbed into the system, and the brakes had a complete overhaul about 2years ago, new lines, wheel cylinders, etc, shoes and drums are good.

So my question is this - should I be looking for a problem, or was the hard pedal a result of the way I was braking at that point in time.
If I was idling, heading downhill, did I simply 'run out of vacuum' , and I should have been in a lower gear and not dragging the brakes.

Cheers

[Phil B]

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.

[Ozdunc]

Cheers Phil.

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

[Blknight.aus]

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.

[Phil B]

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.

[Blknight.aus]

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)

[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?

[Blknight.aus]

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.

https://www.aulro.com/afvb/images/imported/2014/03/9.jpg

https://www.aulro.com/afvb/images/imported/2014/03/10.jpg

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.

[JDNSW]

......

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

[Blknight.aus]

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

John

happy to take that but I'd kind of hoped hed notice that and it would have been there as a symptom...

Even then... Id have expected some assitance to be on hand or to redevelop once the brakes were in hard and he stopped jiggling.

but if you want to discount that factor try this...

start the engine, run it at high idle then shut it down. Then depress the brake pedal till it reaches the limit of travel under reasonable pressure (and you're going to have to be a bit guessy here I usually do this with the "my knee got to this far down the steering column cover" routine) measure the position of the pedal.

get onto a hill and start rolling down the hill with the engine off but in gear (2nd or third) push the clutch in and hold it then use the brakes till you have a firm pedal up near the top of its stroke (but dont completely stop the vehicle) Ride the brake pedal again and start the engine on the key the pedal should now sink and if you maintain the same effort (not position) on the pedal the vehicle should maintain the same speed. If you lift up a couple of inches and apply the same pressure the vehicle speed should drop off and the pedal should not sink quite as far as it did before. (you can let up off of the clutch but dont forget to account engine retardation in the braking effort. if the vehicle passes this test doing it again at higher speeds and letting he engine build up higher vacuum against the closed throttle is worth trying to help eliminate engine vacuum problems)

Failure for a partial reset of the brake pedal to debelop brake assistance indicates the failure of the poppet to seal in other than the fully released position.

IF at any time during this test the pedal sinks lower than the height you first noticed STOP, drop the clutch in and bring the vehicle to a halt.

In the event of the brakes going below your static test height prior to all of the testing (when you had a full vacuum developed on the booster but the engine off) you have more than one fault within the braking system which could be as simple as a combination of low vacuum availability from the engine for whatever reason and a slight leak of the poppet when its in a holding postion right up to you have a leak in the diapragm at certain postions and the MC is recirculating on one circuit or in certain postiions and is requiring extra stroke depth to compensate.


One other thing I forgot to mention is it could also be as simple as there is not enough "port size" for the engine to draw the vacuum on the booster fast enough at low engine RPMS.

[JDNSW]

happy to take that but I'd kind of hoped hed notice that and it would have been there as a symptom...
........

"I'm covering the brake and pushing down on it every now and then"

John

[Phil B]

Ozdunc,

Two completely different opinions here but that's why it is called a forum.
your call...

Phil.

[Ozdunc]

Alright, I ran the test to find out whether the MC is pulsating.

With vac plugged, I drove up and down the road, brakes were nice and smooth no pulsing or grabbing.
Vac is 21-22inHg at idle with a nice steady needle.

Then just for kicks I reconnected the vac and did an emergency stop from about 20km/h - it practically stood on its nose.

So either is my dodgy driving - I don't think I was jiggling my foot on the brake, more like slight pressure, quick increase in pressure, back to slight pressure, quick push, back to slight, etc without releasing the pedal fully. (Although to be fair up until the pedal went hard I wasn't concentrating on the brake system, but merrily chatting away to the missus. Even then I hid my surprise well, don't want to give excuses to take her car out on errands:wasntme: )
Or its something in the booster. My booster looks different to the example you gave above Dave, in that there don't appear to be any mechanical pushrods, just fluid in/out with what looks like an additional MC attached to the booster

https://www.aulro.com/afvb/images/imported/2014/04/1307.jpg (https://flic.kr/p/mDkHRu)
From the top
https://www.aulro.com/afvb/images/imported/2014/04/1308.jpg (https://flic.kr/p/mDje8P)
from the bottom in front of the firewall.

I presume this design would still have a valve and poppets, but would it be more liking to be leaking fluid if there was a diaphragm/rubber seal failure?

[Phil B]

That is a remote VH44 type booster the same as I have on my series 3.
That may account for the issue but I'm sure Dave will give us more advise.

Phil

[Ozdunc]

I'm valuing all the opinions.

If just turns out to be my driving, I've learnt an absolute heap about testing brake boosters.

And if it turns out be the booster/vac, I'm a better driver than my massively inflated ego already thinks I am.:D

Cheers

[Phil B]

I couldn't agree more !!
I am exactly the same boat.

Phil.

[JDNSW]

The remote booster works exactly the same as the explanation Dave gave, except that instead of a pushrod you have a column of hydraulic fluid from the master cylinder to the booster.

It still comes down to your having either a usage problem or a lack of vacuum, either due to a leak, a defect in the booster, or a restriction in the vacuum line. It also occurs to me that you would get a reduced vacuum if you have the engine idling too fast.

John

[Ozdunc]

Thanks John,

With that, the results of Dave's initial tests and the fact they're working great now I'm going to take on a blow to my ego and assume dodgy driving technique.

If it happens again I'll do the rolling hill tests and go from there, at least knowing the booster model means I can get parts or get it refurbed if necessary.

Thanks everyone, esp Dave, for your help.

[67hardtop]

Hi, I think u will find it was your driving style that caused the problem with the brakes. Try fitting a remote vacuum reservoir into the system. it will give you a greater capacity of vacuum for the braking system. You can hide it under or behind the left side front guard or above the left footwell area. Cheers Rod

[Blknight.aus]

might also pay to check and clean the filter thats screwed in under the MC on the booster in your last photo.

Brakes get very interesting when theres no external air getting in through that....