Sinking brake pedal, not master cylinder

[moodybluetwo]

If one of the calliper slides has seized when you apply the brakes the calliper tends to bend toward the free side…..when you take the pressure off the brake pedal the calliper bends itself back ….and goes back just enough to cause extra travel on the brake pedal. It happened to me twice…both on Toyotas….both front offside wheels. They also failed the rolling road brake test. Were your pads on the seized calliper worn uneven? If so this will give you an idea of the extra travel needed to apply the brakes.

[jasper5]

If one of the calliper slides has seized when you apply the brakes the calliper tends to bend toward the free side…..when you take the pressure off the brake pedal the calliper bends itself back ….and goes back just enough to cause extra travel on the brake pedal. It happened to me twice…both on Toyotas….both front offside wheels. They also failed the rolling road brake test. Were your pads on the seized calliper worn uneven? If so this will give you an idea of the extra travel needed to apply the brakes.

The OP said the pedal was solid though with the servo empty.

[moodybluetwo]

In my case the pedal was solid with no servo assistance….this was due to the fact that without assistance there was not enough pressure to bend the calliper. With the extra pressure available from the servo the calliper bent to one side and that accounted for the extra travel.

[RobTurbo]

Just fitted a brand new OSR caliper, everything still feels the same, when I press the pedal it starts as a hard pedal that doesn't slow at all, then starts sinking and the brakes work, as if there's a delay in the servo working, but as soon as it starts working the pedal sinks to the floor. Time for a new servo I think, but it will have to wait until next month or later now.

[moodybluetwo]

It may be a good idea to try the brakes at a MOT centre. If the brake readings on each axle are similar then that may point to a fault elsewhere.

[impster]

Mine does this too.

I've got a very very slight pull to the left occasionally when I brake in a straight line.

Going to put new discs and pads on it very soon, so will check the calipers at the same time. I assume that the calipers are '2 pots' (2 pistons each caliper) yes?

Impster

[steve_earwig]

I think they're one, although it's a long time since I did the brakes. Umm, does it matter? I'd think you need to head for the sliding pins 'cos PSA was a bit sparing with the old grease

You know, the more I think about this one, the more I can't see it being the servo. Last time I had one to bits all they did was use the vacuum to pull a diaphragm forward to assist the brake pedal. I can't see it loosing fluid that way.

Did we do air in the ABS block? What about a bulging brake hose?

[drjonts]

Interesting reading folks.

My 53 plate 110 HDi has just demonstrated something similar - noticed it whilst trying to check brake switches to get cruise control working. Now and again with the engine running and slight pressure on the pedal it seemed to just 'sink' away under foot. Car was parked with the handbrake on so I don't know if it does it whilst underway. Pedal is hard under foot with engine off and all vacuum assist pumped away so don't think it's air in the system. Doesn't seem to happen every time but it may have only just started.

Have read on other searches about someone needing a servo (don't know the mode of failure), vacuum leaks, a certain vacuum valve failing and others having no fluid leaks visible or externally, but an internal leak on the master cylinder - presumably a seal bypass which allowed fluid to pass backwards towards the master cylinder reservoir once under pressure and helped by the servo. The resulting fluid diplaced causing the pedal to travel further downwards as the master cylinder piston travels. Is this a possible explanation? Are there front and rear circuit ports on the master cylinder and it could be fluid bypassing from one circuit to another. Presumabaly once released, the fluid levels all revert and ready to go again. Means that braking effort cannot be as good as without the bypass but it's not leaking out. I guess that's not the case here as the MC is a new one.

Bulging hose also comes up - so might be my first port of call at this point.

Thoughts???

Jonty

[drjonts]

Only me!

Just read this elsewhere...

First of all if it is a Diesel with ABS you will get pedal creep when pressing the pedal without driving.
Does the pedal creep without the engine running if not then you do not have any external fluid leaks .
If it does creep without the engine running then yes you will have an External leak.
Please also check does the pedal creep whilst braking when driving if it does not then YOU DO NOT HAVE A PROBLEM.
You see you could be trying to fix a problem that is not there . I consulted Peugeot Technical on a 605 over this matter and after changing everything they told me DIESEL ABS WILL HAVE PEDAL CREEP


Not sure if this is RobTurbo's problem as I think it happens whilst driving for you, but it could be this is what certain cars do?

Anyone have any feelings on this?

Jonts

[impster]

Hi thanks for these responses. I'll keep an eye on this just in case.

[bangernom1cs]

I have a similar problem.
Looking at the servo I can see where (I think) some leaking fluid has run down the front face.
I had a similar problem witha Montego years ago and turned out to be an internal leak on the master cylinder. ended up changing both...The servo was full of fluid!....just like I'm going to have to do on mine.

T

[RobTurbo]

Been a while since I was on here, I'd learned to drive with iffy brakes, they're not that bad, it passed an mot, but I'm still not happy with them, so I've finally bit the bullet and bought a new servo, brand new genuine pug part, I'm hoping it arrives before the xmas holidays so I can fit it while I'm off work.

If it fixes the brakes I'm going to get cracking with the siemens to bosch management swap, I have all the bits I need I think!

[steve_earwig]

Christ, I dread to think how much that cost I hope it does the business!

[RobTurbo]

From pug the servo is £121.46 according to service box, mine was £70, new old stock, if it doesn't cure my problem the only other thing to change is the abs hydraulic block, I have one from a 306 which I believe is the same, just with a different ecu, I've already removed the ecu from it ready to fit if I need to, but I don't think it's the problem.

[madmadmax]

you may be trying to fix a problem that's not rely there sinking peddles are part of owning a modern diesel

have you checked all the brake calipers are free?