Brake pedal creep

[itiejim]

I've recently noticed that my brake pedal seems to creep - ie, under steady braking pressure the pedal moves, an inch or so I suppose, though it reaches a point at which it stops (reassuringly)!

The car brakes very well indeed, but I wondered if this was likely to be a leak in the servo?

There's no noise associated with this and no other indication of bad running that you might expect with a vacuum leak, any ideas - or are they all like this?

[GreenV8S]

Perhaps a worn master cylinder. Does it also do it when the engine is off and you've exhausted the servo vacuum? That would eliminate any effect of the servo being slow to build up vacuum.

[itiejim]

No, the hydraulic part of the system is fine. it's just the "lighter" part of the pedal feel which seems to creep, then the pedal reaches the point at which it meets true (rather than assisted) pressure and stops.

I suspect it's a slight leak in the servo or pipework.

Anyone know where the servo is - attached to the master cylinder or remote?

I guess it's going to be one of those head in the footwell jobs

[Brummmie]

Sounds master cylinder to me also. You could pull the servo pipe off completely and test though.

[itiejim]

Thanks for the input - how can I best get at the servo?

[copilot]

Sounds like master cylinder - Check if you can see contaminents in the master cylinder reservoir. You need to change the fuid regularly as it is highly hydroscopic (draws in water) which starts the rot.

[itiejim]

I've tried the brake pedal without the engine on (so no servo assistance) and it's rock solid, no creep at all. Also the master cylinder is completely full of fluid, so I'm satisfied that the hydraulic system is sound and that it's not letting any fluid past.

So, maybe a slight air leak in the servo? I'll have to investigate again tomorrow.

[GreenV8S]

itiejim said:

No, the hydraulic part of the system is fine. it's just the "lighter" part of the pedal feel which seems to creep, then the pedal reaches the point at which it meets true (rather than assisted) pressure and stops.

I'm not sure what you mean by that. The pedal creeps under moderate forces but not under heavy braking? That might indicate that the part of the m/c bore corresponding to normal light braking has been worn/damaged allow the pedal to sink slowly. If so you would expect it to sink without the servo too, although the pedal force would need to be much greater to get the same effect without the servo assistance.

[itiejim]

I think it's just normal behaviour you know.

The hydrualic system is definately fine, under heavy pedal pressure with the engine running there is no creep.

The thing that struck me as odd is that, under light but even pressure the pedal slowly falls away, about an inch, until it reaches a solid "stop".

Fundamentally, I know that there is no problem with the hydraulic system as it's not losing any fluid and the brakes are massively powerful so I've come to the conclusion that it's just a feature of the servo setup.

[Brummmie]

Mine does not have this......

feature.

[GreenV8S]

itiejim said:

Fundamentally, I know that there is no problem with the hydraulic system as it's not losing any fluid and the brakes are massively powerful

Neither of those are proof that the hydraulic system is OK.

itiejim said:

so I've come to the conclusion that it's just a feature of the servo setup.

You're probably right.

[itiejim]

Well, the car brakes very well indeed, doesn't lose fluid and has a rock hard pedal except for in the circumstances described, when it becomes rock hard after about an inch of sinkage. I can't find any indication of an air leak so... I'll suck it and see (so to speak).

Thanks to all for the input.

[Brummmie]

Clamp the pipe from the, I'm assuming inlet manifold (i dont know how the AJPees are set up) to the servo, if your pedal still creeps then you know this feature is in your hydraulics, i would be tempted to find this out sooner than later.

[GreenV8S]

Given that you've tried without the vacuum and there's no problem, I think it's most likely a servo/vacuum issue. Since you have throttle bodies the vacuum source is probably quite restricted so may not be filling the servo quickly, that might explain why it takes a while to build up full vacuum. I think you're right to put it down to a quirk of the car and not anything to worry about.

[Brummmie]

He says no creep under heavy pressure when engine running, does it still creep under light pressure when the seals have less pressure against them to assist the seal? if you see what i mean.

ETA reading through it seems he has no creep without the engine running, so in which case agreed, its more than likely ok!

Edited by Brummmie on Friday 20th February 10:07

[baztvr]

It's often the case that servo brakes will creep when you rest your foot on the pedal. I've had opel mantas they all did it,my xj8 did the same. never had an actual problem with the brakes though. and as Brummie said, if you stamp on the brakes, the seals hit pressure and helps the seal to 'flare' out therefore pushing harder against the cylinder wall= no fluid creeps around the sides of the seal.