brake pedal drop

[GTIR-LOZ]

has anyone ever solved the problem of where you turn a corner and then push the brake pedal and on the first push it sinks to the floor, second is fine??

i find lots of references to it on a search but no one seems to have found a reason

ive just put my hispecs on and pressure bled them and definatley no air was comming out

take it for a drive and still the pedal drops on the first push its actually worse than before i.e just manouvering in a car park can loose the brake pedal

the pedal feels totally solid with no spongyness and feel really great when they are working

there is no play in the wheel bearings or any suspension components

the car has braided lines all round

could this be a master cylinder fault if so why?

i could understand pad knockback if i was going fast but you can turn out of a junction crawling and when you go staright there is no pedal again its quite dangerous as well as annoying

 

[Fast Guy]

The only things that I could see causing that are there is still air in the system or the pistons are retracting slightly everytime they are used so they need to come out before anything happens.

I thought pad knock was the pad moving in it's mounting slightly in the direction of wheel rotation, not moving in and out in the direction of travel of the caliper piston?

Try jacking the car up with the front wheels off and the engine running, get someone else to press the brake pedal (and let you know if there's no brakes on the first push) and see if you can see anything untowards with the pads/ calipers.

 

[GTIR-LOZ]

no pad knock back is where the disc pushes the pads back into their pistons moving the pistons back hence giving you a longer pedal travel

the problem is that you have to be moving for the pedal to drop it does not do it at standstill

i dont know any better way than pressure bleeding the brakes, if there is one i will try it

 

[Fast Guy]

Wonder if the brake discs are out of spec on run out?

You do have your calipers on the right way up? bleed nipples at the top?;-)

In fact check for anything that could allow the disc to move relative to the cailper- wheel bearings, ball joints etc.

 

[Braveheart]

I can't advise of the fault but I find that gravity bleeding the brakes gives me good results every time. Here's my method though;

Make sure you keep the reservoir topped up at all times.
You can try just bleeding the front first and see what the paddle is like.
When I upgraded my brake. New front four pots and braided lines and fluid, I just gravity bleed the lot. I quickly blead the air out the front then gravity blead all four.

Opened the bleed nipple(you can even screw it right out) on n/s rear caliper, gravity bleed (i.e. without pressing brake pedal just let the fluid seep/flow out) until the new fluid started to come through then tighten bleed nipple back up. Do the same for o/s rear, n/s front then finally o/s front.

If you have changed pads and not disks, the pedal will feel a bit sloppy/spongy until the pads bed in to the old disks.

 

[GTIR-LOZ]

fast guy they are all brand new and i get no vibrations they have 4 bleed nipples so the caliper can go either way up, also they did the same on my old brakes so i dont think its the actual brakes, the only things that can move are the hub or the wheel bearing both have no play in them i am going to order a master cylinder reseal kit just in case its that

braveheart, i might try that one but i would of thought not all the air would get out i will try a re-bleed monday

 

[pulsarboby]

i would say master cylinder loz!
it could be a worn pressure seal thats causing it.

sit in the car with engine idling and apply pressure to the brake pedal for around 30 seconds and see if the brake pedal slowly starts to sink to the floor, if so then the pressure seal is knackered

 

[GTIR-LOZ]

i did try that bob but it comes up fine

 

[JoeGT]

From what you say I reckon you have a vacuum leak in the brake booster.. I had this a while ago in my 4WD P10 Primera.. I had taken the brak booster in for some coating as part of the "pretty up" in my engine bay.. Once it came back and was refitted, I had the same issue once the car was started.. Def easier to replace the brake master before going for the booster but I found as soon as I replaced my booster all of my problems went away..

 

[GTIR-LOZ]

i thought if you have a faulty brake booster then the pedal would go hard if the master cylinder does not work then i will try that next

 

[GTIR-LOZ]

well its not the master cylinder i changed that tonight and its still the same i have been noting when it happens and its always on right hand bends, which leads me to think its something on the n/s of the car

also when i was bleeding the brakes the o/s/r and n/s/f bled really slowley hardley any fluid was coming out while the other 2 calipers it was really quick and was bled inabout 2 minutes does the proportioning valve control the brakes like this or is it a front rear split

 

[pulsarboby]

just sent you a pm loz
reckon i know whats causing prob now;-)




just read whole post and youve just hit the nail on head buddy! the valve is diagonally split

 

[GTIR-LOZ]

cheers bob i replied:-D

 

[Davie Houston]

Need to let us know if bob has sussed it out.
Just in case i need to change the anything before i fit my 4 pots.

 

[DanDud]

Iv had this problem for ages now, only really happens when iv been cornering at high speeds.

What is it Bob?

 

[Animaldaz]

the proportioning valve on the bulkhead.

 

[GTIR-LOZ]

its only a posibility i will put one on at the weekend and let eveyone know, if it doesnt fix it i will change the wheel bearing on the n/s/f and try again

 

[pulsarboby]

as loz and animaldaz have pointed out, it could well be the proportioning valve.
my guess is that its either airlocked or something inside it has broken down or stuck!

wont know for sure till hes tried another one

 

[vss irvine]

could it be worth changing the flexi hose first? proportioning valve is a big job as the inlet manifold needs to come off!

 

[GTIR-LOZ]

all the flexi hoses are newish and it did that before, i now think its either the n/s/f wheel bearing or the c.v joint as the wheel hub does not seem to run true when the car is on the ramp, there is still no play but you can see the disc/hub/c.v joint wobbling slightly i have all the parts now so will hopefully put them all on sat