Technical Funny brake pedal business

[Jimbo34]

Hi
Front pads were well down, almost to the plate and the front discs well worn, so out they came.
Opened the bleed nipples to avoid flipping a master cylinder seal, and pushed the pistons back in, new discs and pads. Torqued the bolts to the right torques in my book. Bled the four corners with a helper in the prescribed order. The pedal was nice and hard at the end of the bleed...
So I shut up the shed and drove her out.
With the engine running the pedal was suddenly noticibly softer and sank two inches biting progressively. The travel feels deeper and softer than before the disc change.
If I pump the pedal before starting the engine it is hard and stays high. If I keep my foot on the brake pedal and start the engine it drops away and stops about half way down to the floor.
On the road the pedal seems to go down further than in my memory but braking performance is very good and progressive, at all speeds. I can hold the car on the brakes on a slope.
Just softer, mushier, than I remember. Reminding me of a failing master cylinder on a Renault 4 I once had. Although here I dont have to pump to get good braking...
Driving a Citroen Xantia all week with its fantastic ON/OFF style brakes makes the pedal contrast even more pronounced.
I cant see how changing the pads and discs would change the biting point on the pedal travel, or is it in my mind ? I doubt I there is any air either I put half a litre of DOT 4 through each corner with no bubbles at the end.
Any ideas ?
I am going to drive it softly and watch for a change in performance. I am worried it is a master cylinder problem because of the sinking pedal thing, although I might not have notice it before, the missus drives the Multi more, but it all works fine and I dont have to pump the pedal...

 

[Ffoxy]

I'm pretty sure the brakes always feel less crisp when pads & discs are swapped, give it a hundred miles or so & I'll bet the brakes will feel fine.

The sinking pedal behaviour on start up from a hard pedal is normal.

 

[QinteQ]

Hi
Front pads were well down, almost to the plate and the front discs well worn, so out they came.
Opened the bleed nipples to avoid flipping a master cylinder seal, and pushed the pistons back in, new discs and pads. Torqued the bolts to the right torques in my book. Bled the four corners with a helper in the prescribed order. The pedal was nice and hard at the end of the bleed...
So I shut up the shed and drove her out.
With the engine running the pedal was suddenly noticibly softer and sank two inches biting progressively. The travel feels deeper and softer than before the disc change.
If I pump the pedal before starting the engine it is hard and stays high. If I keep my foot on the brake pedal and start the engine it drops away and stops about half way down to the floor.
On the road the pedal seems to go down further than in my memory but braking performance is very good and progressive, at all speeds. I can hold the car on the brakes on a slope.
Just softer, mushier, than I remember. Reminding me of a failing master cylinder on a Renault 4 I once had. Although here I dont have to pump to get good braking...
Driving a Citroen Xantia all week with its fantastic ON/OFF style brakes makes the pedal contrast even more pronounced.
I cant see how changing the pads and discs would change the biting point on the pedal travel, or is it in my mind ? I doubt I there is any air either I put half a litre of DOT 4 through each corner with no bubbles at the end.
Any ideas ?
I am going to drive it softly and watch for a change in performance. I am worried it is a master cylinder problem because of the sinking pedal thing, although I might not have notice it before, the missus drives the Multi more, but it all works fine and I dont have to pump the pedal...

Ffoxy is right, half an hour or more and they will ' bed in '

You can't compare the two on pedal pressure. The Xantia is under extremely high pressure from the accumulators into the brake lines.

 

[kev78]

Agree with both the above replies. Xantia cannot be compared at all as the system is not conventional by any means.

The action of the pedal dropping when the engine is started is exactly what should happen as the brake servo kicks in and assists your leg muscles to apply more force to the brakes.

New brakes always feel a bit mushy during the bedding in period.

As long as the brake pedal isn't creeping to the floor then it should be just the bedding in

 

[Jimbo34]

I hoped so too.

I have done a 100 miles and it is just the same. The pedal now nearly sinks to the floor I have discovered if I press really really hard.

I took it all to pieces again and bled it. Twice. No air bubbles.
No change.
If I bleed it, rock hard pedal. Engine on and it is soft and sinks to the floor.
Pumping it picks it up.

Master cylinder ?

:bang:

 

[matt8772]

how many miles have you done since you changed the front d+p? enough for them to be bedded in?

the reason the pedal sinks to the floor is because of the servo assistance.

when you next drive it try a bit of left foot braking and see if that hardens the pedal up a bit.

out of interest if you pull the handbrake all the way on does the feel of the pedal change at all?

if youre still not happy with the feel of the pedal after letting the brakes bed in for 25/30 miles try clamping each one off in turn and seeing if the pedal feel changes.

 

[Ffoxy]

I hoped so too.

I have done a 100 miles and it is just the same. The pedal now nearly sinks to the floor I have discovered if I press really really hard.

I took it all to pieces again and bled it. Twice. No air bubbles.
No change.
If I bleed it, rock hard pedal. Engine on and it is soft and sinks to the floor.
Pumping it picks it up.

Master cylinder ?

:bang:

Leaky servo hose?

 

[fletch101101]

That is perfectly fine, old style drum brakes had a self servo action and when they are applied they pull them-selfs into the brake drum due to the rotating force and the wedge shape on the back. but modern disc brakes dont have this so need a brake servo to give the required brake pressure needed.

The feeling you get of the pedal going down is jsut the servo getting pressure and you are unconsciously testing the brake servo.

The brake servo uses vacume pressure from the inlet manifold to help you brake. so that you experience is perfectly normal

Just a little bit of advise if that you dont need to crack off the bleeding nipple when changing pads of discs, just push them in. just take the cap off the brake reservoir to stop it squirting up if it reaches the top

You wont damage your master brake cylinders because when you lift off your brake, at the end of the brake cylinder's travel it uncovers a little port leading to your brake reservoir so any fluid will just flow up into that. this is how the brake fluid goes down as the brake pads wear, they draw in the excess fluid they need. this keeps the brake pedal travel the same even when they wear

 

[fletch101101]

I hoped so too.

I have done a 100 miles and it is just the same. The pedal now nearly sinks to the floor I have discovered if I press really really hard.

I took it all to pieces again and bled it. Twice. No air bubbles.
No change.
If I bleed it, rock hard pedal. Engine on and it is soft and sinks to the floor.
Pumping it picks it up.

Master cylinder ?

:bang:

Hummm check your brake reservoir level and with the engine running, press the pedal hard for about 5times and hold it for about 5 seconds then go and check the brake reservoir, if its the same its all fine but if its gone down, you got a leak. also check all the nipples to see if they are tight, its very easy down to leave one loose :idea:

 

[pete123]

How did you no your brake pads and discs needed changein dont mean to sound like a plank but i got a lot of travel in my pedal and when it does reach to break it slams on cant jack car as fiat jacks are pants so gotta sorce new jack

 

[fletch101101]

Go to halfords and buy a jack and axlestand pack then take off yoru wheel.

look at the brake pads and see how much of the brake pad is left, nto the metal bat plate the actual friction materian, less than half a cm and i would change it. if your disc is groves of badly lipped i would change that to at the same time.

no offence but if you dont knwo how to tell me might be better to take it to a garage


or try and find someone who knows, who can show you, where abouts are you based pete mate

 

[Jimbo34]

Hi all,

Well I changed the pads and disks originally because evey six months I take the wheels off and have a gander at what is going on. This time they were well used...

Back to the game.

Today I put in a shiney brand new master cylinder. Bled the system as normal, pedal nice and hard. Got in the car started the engine and the pedal went all soft and started dropping...

Argh... :bang:

Have I got air in the ABS or something ? I have a vacuum pump for liquids I feel like sucking loads of fluid through at a good speed.

One of you mentions leaks. Well the liquid level stays the same, I will double check tomorrow.

I was wondering about the handbrake adjuster, must read up on that as I know nothing about that.

 

[Jimbo34]

Reading through this theme on other Fiats and RB2009 said this about a Punto which makes me think that it is air in the system.

does it feel spongy? if you continue pushing the pedal and it gets to the bottom and you can still force it a bit more.. and it feels spongy you have air in the system, in which case you will need to bleed the system... another test for spongyness is with the engine running.. dont do it with the engine off because each time you press the pedal the vacuum servo unit is depressurized and it will go firm anyway.... but with the engine running.. pump the pedal a couple of times, the pedal should feel consistant with each press of the pedal.. if it feels spongy, or rises in height each time its pressed then you probably have air in the system.. you may of heard the expression pumping the pedal up... thats a symptom of air and something people with hydraulic clutches sometimes have to do when air gets into the system.. they press the clutch and it doesnt disengage so they have to pump it a few times to disengage the clutch.

Tomorrow morning I will have a push again.

 

[fletch101101]

the vacume is air not liquids thats why you can hear air noises, i think that your just being paranoid maybe, we all bin there at times,

when you bleed the brakes do you put a tube over the nipple into another container of brake liquid?? try doing that as it will help you to not get air in and also stop you making a big mess.

if your not using a tube when the person is pressing down and you crack off the nipple remember to close it again before the squirt stops not as it stop if that makes sence

 

[Jimbo34]

Hi Fletch
Yes, I have someone help me too. A tube in fluid in the bottom of a jamjar, I say "down", they pump down gently, I lock off. I say "up" with nipple closed and never let the tank level drop either.
I have bled it so much that the fluid is constantly clean and no more bubbles the last 2 bleeds.

 

[pete123]

Poped it up on a jack from halfords and pads and discs are spot on all i found was a piece of like metal wire around my pad not to sure how it got there but it had been there a while removed and brakes are alot better all i had trouble with was really geting it off the floor witch is a pain due a service next week so sure they can check aswell thanks for replyin

 

[fletch101101]

then i think its just the power of the brake servo making you think something is wrong and you have never noticed it before as you have never tried it, i think anyway


another idea is to find someone with the same car and try pressing there brakes

lets hope you got an honest face and people trust you lol

 

[fletch101101]

Poped it up on a jack from halfords and pads and discs are spot on all i found was a piece of like metal wire around my pad not to sure how it got there but it had been there a while removed and brakes are alot better all i had trouble with was really geting it off the floor witch is a pain due a service next week so sure they can check aswell thanks for replyin

glad to hear it pete, its always best to be on safe side, they will check the brakes as part of the service,

 

[Jimbo34]

Rolled the car back and forward on the clutch in the garage using the brakes, fresh this morning and thought I would take her out onto the drive.
Moving with a light foot on the pedal and the brakes felt fine this time !
Rolled down to the road and performance was perfect.
The sinking was still there but quickly stops at a hard point, and I need to push hard to sink a bit.
In normal driving the brakes are progresive and work with little pressure.
If I emergency brake they go down further and bite hard, no pulling to one side.
I think my master cylinder was getting a bit sloppy/terminal after 160 000 km and as I drive the Multipla like a granny I had never really noticed the sinking issue as I seldom push hard. Or the start up pedal sink thing, who does that after all !
I was amazed at the amount of crud I could see inside the master cylinder from the pipe union holes. I change the fluid every two years... just goes to show !

Type in "Brake pedal creep" into the search function on this forum or indeed ooogle, and the things that come up repeat my experience. Its all related to the brake servo and is normal. Up to a point

Thanks for your support !

Ciao

 

[fletch101101]

I said that the brake servo makes it do that lol, glad to hear you have it sorted now mate, never heard of anythig like that before but always learning