Soft Brake Pedal

itarumaa · June 22, 2011

Hi,

how are you all,

just wondering, my Ls400 has a soft brake pedal when engine is running. When the car is in P, the pedal is soft, when in D, then it is soft but when pressed it starts to raise from the floor, also revs drop slightly when running in d and the pedal is pressed.

Brakes work fine, there are no leaks and system has been rebleed 2 times.

It is only that pedal is bit too soft that it should be.

Thanks.

beavisrules · June 22, 2011

bulging flexi pipes or leaking seals in the servo would be my first guess without more info.

like , does the pedal drop if you keep pressing realy hard ?

do the brakes come on instantly if you jab the pedal and gradualy if you press gently.

does the ABS work properly.

remote chance, but could also be linked to a vaccum leak on the servo side and/or dodgy vaccum pump if the LS uses one.

itarumaa · June 23, 2011

bulging flexi pipes or leaking seals in the servo would be my first guess without more info.

like , does the pedal drop if you keep pressing realy hard ?

do the brakes come on instantly if you jab the pedal and gradualy if you press gently.

does the ABS work properly.

remote chance, but could also be linked to a vaccum leak on the servo side and/or dodgy vaccum pump if the LS uses one.

Hi,

when stopped in traffic lights in Drive, and the pedal is pressed hard, it goes to floor. If if hit the pedal down fast, car stops well and I think abs is working, no errors and only slight front wheel lockup, but nothing like it would be with ordinary brakes. VDC works as well.

I think since the revs of the engine drops slightly when in D and hitting the brakes, this mighth point to vacuum leak?

Cheers,

steve2006 · June 23, 2011

With the engine off pump the brake pedal several times,the pedal should rise and become very hard. With your foot still pressed hard on the brake pedal start the engine. You should then feel the brake pedal fall under your foot indicating the servo is working and that you therefore have vacuum to the servo.

If this checks out OK you need to recheck all your brake components for leaks (there have been cases of fluid leaks into the servo which is then sucked into the engine and burnt),bad seals and air.

To check individual calipers clamp off the flexible hose to the caliper and then see if the brake pedal travel is any better,if it suddenly improves investigate the caliper that was clamped at the time.

Did you pressure bleed the brake system? Did you also bleed the ABS and TRAC if fitted?

PS a vacuum leak on the servo would not result in excessive pedal travel all this would do is make the brake pedal harder to press down to apply the brakes.

itarumaa · June 23, 2011

With the engine off pump the brake pedal several times,the pedal should rise and become very hard. With your foot still pressed hard on the brake pedal start the engine. You should then feel the brake pedal fall under your foot indicating the servo is working and that you therefore have vacuum to the servo.

Hi, car is just like this, except when engine is started the pedal seems too soft.

If this checks out OK you need to recheck all your brake components for leaks (there have been cases of fluid leaks into the servo which is then sucked into the engine and burnt),bad seals and air.

To check individual calipers clamp off the flexible hose to the caliper and then see if the brake pedal travel is any better,if it suddenly improves investigate the caliper that was clamped at the time.

Did you pressure bleed the brake system? Did you also bleed the ABS and TRAC if fitted?

PS a vacuum leak on the servo would not result in excessive pedal travel all this would do is make the brake pedal harder to press down to apply the brakes.

There has not been any leaks in 2 months time, so for me it is hard to belive that there would be a leak in the system. The brake fluid has stayed in same level for past two months and maybe 1,5k miles.

But of course your idea about clamping the hose is good one, easy to test as well. The system has not been pressure bleed, only manually. Question: Do I have to pressure bleed the Brake Actuator or can I just bleed it manually?

And thanks for advice.

Mybikesacx500 · June 23, 2011

My soft brake pedal turned out to be a stuck caliper- one of the floating pins has seized and the caliper was twisting, causing uneven wear on the pads.

I found this after bleeding the whole brake system twice, so worth just having a good look at each caliper and checking they are free

ROYT · June 24, 2011

If there are no leaks, [ no loss of brake fluid] if the pedal is soft there is air in the system

If you bleed it again use new fluid,don't shake it up!

itarumaa · June 24, 2011

Works now, I connected the bleeding bottle to Brake Actuator, opened the valve and pushed the pedal couple of times.

Then I put a rock in the brake pedal, and opened master cylinder valve.

This with engine running and now the brakes are fine.

thank you for advice.

itarumaa · July 3, 2011

ok, so now the situation is better, but there is still problems. Sometimes the pedal goes soft and if I then push it to the floor (in traffic lights) then after that brakes are great, for a while.

So maybe tomorrow, after work I should rebleed the system again, all the wheels etc.

But after this, if it does not help, maybe I should get second hand master cylinder?

This because the problem started some time ago, when brake caliper failed and in that situation the pedal was pressed to the floor many times, so maybe the seals were damaged in the master cylinder?

steve2006 · July 4, 2011

I heard of cases where when changing bake pads instead of opening the caliper bleed nipple 1st the pistons have just been pushed back,this can cause the master cylinder or other seals to twist due to the reverse flow direction on the brake fluid and cause problems.

Since this problem started after sorting out the caliper it could be the cause.

It still sounds like you have air in the brake system still.

itarumaa · July 5, 2011

Ok, I bleed the system again. I put the ignition on, when bleeding the rear brakes and that did seem to do the trick.

Then the front one, I connected the bottle to the valve and started to bleed the system, when brake fluid started to leak inside the alloy.

So I tought that now the problem that I had before was repeating itself, but anyway I continued and bleed the another side in the front, then bled the leaking side again.

Pedal was much better, a bit soft still but much better.

So I washed the alloy, and went for test drive.

Brakes now reach much sooner than before and pedal is not so soft, if I rest my leg in the pedal in traffic lights, pedal is ok, if I put alot of pressure then it slowly starts to sink.

I brake as hard I could from 70-80km/h and car stopped fine, a little bit of tyre squell but compltely straight, so I assume abs is ok.

I came back from test drive, no leaks in alloy or no missing fluid and brake pedal is stil quite ok.

So now I am confused, was the caliper that failed and leaked the fluid or could it be the bottle connection that failed, because I did put quite much pressure in the system.

I am a bit confused, because if the caliper would be damaged now, it should leak the fluild all the time or at least in emergency braking, but it does not.

itarumaa · July 5, 2011

Of course I could try to get second hand caliper, but is there really a leak or not? The caliper was repaired before,because it was leaking so that is the reason I am not too sure.

And that time I leaked like that, 5 pedal push and reservour was empty, now no leaks at all.

steve2006 · July 5, 2011

I assume you are using one of those "one man operation" brake bleeders, normally a rubber tube which fits over the bleed nipple and has a one way valve to allow the fluid to come out when you push down the brake pedal but does not allow air to be sucked back in when the pedal is let up.

itarumaa · July 5, 2011

Yes, that I did use, one man bleeder, basically a bottle,hose and a valve.

Mybikesacx500 · July 8, 2011

Yes, that I did use, one man bleeder, basically a bottle,hose and a valve.

If the pedal slowly goes to the floor when being pushed down, and the fluid level is not dropping, then I think the only explanation is it is leaking past the master cylinder seal

itarumaa · July 13, 2011

So I did bought second hand master cylinder, but since the car works and there are no leaks, I am going to postpone the repairs, because I am taking the car with me to Finland (from Ireland), starting of next month. So no point to fix it now, if something goes wrong I am in trouble.

Thanks for the advice so far.

itarumaa · July 15, 2011

Works now :)

I asked help from my friend, who is from Latvia and really good fixing cars.

So he bled the system manually (I was pressing the pedal), engine running and still same. So replacement master cylinder was fitted and repeat: master cylinder, abs unit, rear left, rear right, front left and front right, then abs unit again.

Now the brakes are just fine:)

Thanks for advice, even my friend did not believe that master cylinder could be faulty, but it was.