knock sound when braking

[georgeonboost]

hey guys,

when i apply the brakes in my vp commodore there's a knocking sound coming from under the drivers seat side of the car. it only happens when i apply the brakes and not constantly whilst I'm braking. its got me stuffed as to what it might be. i had a look under the car but didn't notice anything out of the ordinary.

also, when i apply the brakes, the pedal fades to the floor a fair bit as if there was no fluid in it. other times its fine and the pedal feel is awesome! there is fluid and the all the pads are fine. master cylinder has been replaced last year. should i bleed my brakes? or is the booster stuffed?

any help would be greatly appreciated. thanx

[Cheap6]

Perhaps a worn/loose wheel bearing? That would explain the inconsistent brake pedal.

Otherwise check that all the suspension bolts are tight. Also brake caliper bolts and wheel nuts.

Some or all of the suspension rubbers (bushes) may be past it. Lever with a large screwdriver to see if they are torn.

One or more of the suspension/steering joints may be worn. Check for play with the wheel off the ground in the vertical (across the car) and horizontal planes.

[burnz]

have you got the right size pad's....
sound's like exessive play, or as stated loose caliper bolt's..

[georgeonboost]

how can the wheel bearing give an inconsistent brake pedal? im not too sure how that would work out.

the pads are the right size. i know that for a fact. could the pins on the caliper not be greased up and this be part of the problem as i dont think there was much grease on the pins at all when i changed them.

-George

[taskmaster]

how can the wheel bearing give an inconsistent brake pedal? im not too sure how that would work out.

the pads are the right size. i know that for a fact. could the pins on the caliper not be greased up and this be part of the problem as i dont think there was much grease on the pins at all when i changed them.

-George

well wheel bearings can affect braking although it doesnt happen to often

but now a question for you... why ask for help if your just going to turn down the advice as it begins to trickel in??
try re-greasing your pins but ims ure youll find that not to be the problem

[georgeonboost]

i wasnt turning down advice. im sorry if it came across like that. i was trying to ask how that would affect it as im not too sure about this sort of stuff.

ill check my brake calipers etc and if that doesnt fix the problem i guess ill take it somewhere to get it looked at.

thanx again guys.

[Cheap6]

how can the wheel bearing give an inconsistent brake pedal? im not too sure how that would work out.

-George

If the wheel bearings are loose, cornering load causes the disc to move inside the caliper and push the piston and pads away from the disc. The next time you press the brake pedal, the caliper piston has to move extra distance before contacting the the disc = low pedal. If you don't corner between pedal applications the pads don't get pushed back and the pedal stays high.

Edit: This is a better explanation:

http://www.stoptech.com/tech_info/wp_knockback.shtml

[Morton]

Wheel bearings couldn't be easier to check

: remove wheel
: remove caliper
: remove dust cap in the centre of the rotor
: remove the split pin on the bearing nut
: remove the bearing nut (27mm, it shouldn't be tight)
: pull the rotor toward you, it will come out with the outer bearings. Take note of how they fit in
: remove the rear bearing

: inspect all parts, and clean them with a clean rag or good paper towels
: check bearings for faults, broken rollers, cracks etc...
: grease bearings up thoroughly with bearing grease, use your fingers to work it in nicely
: replace rear bearing
: replace rotor
: replace outer bearing and retainer
: ensure it's packed with grease, but that you don't get any on the rotor
: ensure everything is in as tightly as it will fit, and replace the bearing nut
: do the bearing nut up until it is hand tight, then back it off 1/16 of a turn
: replace the split pin
: replace the dust cap
: clean your hands and check that there is no play in the rotor

While you're in there, check the tie-rod ends, swaybar links and radius rods for wear or damage.

[VNHeartache]

For what it's worth, I'd bet on wheel bearings or caliper loose. Definitely something loose there and I wouldn't be driving it too much till it's fixed as you may find your car embedded in the back of someone elses' before you know it. If you're not a mechanic, I like your idea of taking it to someone who is.

[georgeonboost]

i try my best to do as much to my car/with my car as i can but when it comes down to things related to brakes id rather take it to a mechanic. thanx for the info guys. muchly appreciated.

-George

[georgeonboost]

ok guys. bit of an update. i got new front rotors (slotted), wheel bearings and pads fitted today and the brakes were also bled. pedal feel is heaps better but sometimes when i brake is the pedal goes down a bit and othertimes its got a good feel to it. could my booster be stuffed?

and that knocking noise hasn't gone away either. anyone have any idea as to what that might be?

-George

[VrWagz1]

If you hold your foot on the pedal and it slowly sinks, then it could be your booster if you cant see any visible fluid leaking out. Be sure to have a good look under where the master bolts to the booster for leaks too, cause if its there it dribbles down and lands in the engine bay and strips the paint had this happen to me.

[georgeonboost]

master cylinder has been replaced with a brand new item last year. theres no signs of leaking anywhere. looks like im gonna have to change my booster.

-George

[johnnythaboy]

does the brake fluid look clean? alot of the time when there is a leak, the fluid will be dirty looking.

have you had the diff mounts etc checked to see if the noise is coming from the back?

[ScoHar]

master cylinder has been replaced with a brand new item last year. theres no signs of leaking anywhere. looks like im gonna have to change my booster.

-George

Might sound stupid, but check if your booster vacuum line is in good condition, I dunno but if it had a small tear or hole im sure that would case funny braking

[danja]

Mate of mine had a similar thing on his Audi, after lots of dicking around it turned out the problem was someone had fitted the wrong pads to the car!

They were close enough to the correct ones that they fitted and worked Ok, but they were a bit to small and there was some movement on application which produced a "clunk" or knock from when braking.

New pads and was all fixed.

[samuels]

well wheel bearings can affect braking although it doesnt happen to often

but now a question for you... why ask for help if your just going to turn down the advice as it begins to trickel in??
try re-greasing your pins but ims ure youll find that not to be the problem

wheel bearings will not affect your pedal for braking,give the person a break they are only asking for some advise.

[samuels]

ok guys. bit of an update. i got new front rotors (slotted), wheel bearings and pads fitted today and the brakes were also bled. pedal feel is heaps better but sometimes when i brake is the pedal goes down a bit and othertimes its got a good feel to it. could my booster be stuffed?

and that knocking noise hasn't gone away either. anyone have any idea as to what that might be?

-George

how does the pedal feel when you first hop in the car without motor running ?
if it feels good ,next with your foot still on the brake pedal start the car your pedal should move down slightly,you will know as you can feel it,if it goes more than slightly...I.E heaps,then i'd be considering your booster.
as for tracking down the knocking noise check radius arms for tightness,sway bar and tops and bottoms of struts (the bolts at bottom and nuts at top)

[Cheap6]

wheel bearings will not affect your pedal for braking,give the person a break they are only asking for some advise.

I'll assume that's not trolling and suggest you read the link in post #7.

[samuels]

I'll assume that's not trolling and suggest you read the link in post #7.

i'll assume you read the article properly,like it said the world isn't perfect and pedal knockback is more common on the race track.

[Cheap6]

i'll assume you read the article properly,like it said the world isn't perfect and pedal knockback is more common on the race track.

With everything working as designed, a racetrack is the only place that deflection in components will produce noticeable pad knockback. A loose wheel bearing can have a similar effect.

[samuels]

With everything working as designed, a racetrack is the only place that deflection in components will produce noticeable pad knockback. A loose wheel bearing can have a similar effect.

not necassarily,you usually hear the humming ...grinding first,unless your deaf then you will have all sorts of steering problems rather than braking problems,i'm not denying that pedal knock back doesn't exist,but it is very uncommon in everyday normal driving.

[darthdingas]

master cylinder has been replaced with a brand new item last year. theres no signs of leaking anywhere. looks like im gonna have to change my booster.

-George

hey george , one thing with commodore's is the brake's they are known for a slightly "soft" pedal feel.. this might help under the master cylinder there is a switch tells the cylinder how far the poppet valve moves , start you car pump the pedal tell it feels hard turn car off remove plug from switch remove switch " there is a small spring at the top of switch DONT lose it!" clean swicth and spring replace, but again it is a commodore and they are known to feel like there is no pedal for a little bit SCARY i know.