Sinking brake pedal, not master cylinder

[RobTurbo]

My 406 has had a sinking brake pedal since I bought it a few weeks ago, at first the brakes were poor but after a bit of driving they got better (car had been sat apparently), I checked for leaks first, it wasn't loosing fluid and it wasn't leaving puddles so that was ruled out, next on the list is master cylinder, I had one laying about from a 406 v6 so I fitted that, I couldn't bleed all the brakes because the OSR bleed nipple was completely blocked, I had a calliper with one I could use, so I bled the 3 others and tried the brakes, still sinking pedal but a lot better, went and got the other nipple and fitted it, bled the brakes again, now I'm back to square one, sinking pedal!

I read something about the servo causing this, but I'm not going to buy a new servo and mess around changing it until I know how it's possible for a servo to cause this. All the servo does is amplify the force on the pedal, if there's no leak or air in the brake fluid then all the pressure should be going to the callipers, but it's clearly not, so where's the "give" coming from?

[Doggy]

If you pump the pedal multiple times, with the engine stopped to drain any residual vacuum, you should feel what the pedal's like without the servo.

[steve_earwig]

They do that, unless it's dropping straight to the floor of course.

[RobTurbo]

It does sink to the floor, not all the time, if I just brake to slow down or gently stop then they work fine, if I have to stop quickly, not even emergency stop type quick, just quicker than usual, the pedal feels fine at first, then starts sinking and the braking fades, not completely, I do still stop, just not as quick as I'd like, it has slowed my driving to old fart speed though!

If I stop on a hill and hold the car on the brakes, even though the pedal will sink to the floor the car doesn't start to roll, so the brakes are still working.

I know when a servo breaks you usually lose assistance and end up with a rock hard pedal and very little braking effort, this is nothing like that, that's why I can't understand how a faulty/broken servo could cause this like I've read!

[KrisB]

Sounds like you've got a fluid leak somewhere.

Or massive brake fade.

[RobTurbo]

There is no fluid leak anywhere, I've checked and double checked, it has rear disks as well so it's not a rear cylinder leaking into a drum.

And you don't get brake fade when you're not moving.

I tried using pp2000 to bleed the brakes tonight, still exactly the same, I have no idea why but I think I'm going to try a new servo, it's in for an mot at the end of the month so they need to work!

[KrisB]

In that case then, id do a new master cylinder before you do a servo.

[RobTurbo]

I know 100% that the master cylinder I just fitted is perfect working order.

The only parts that I can't vouch for are the servo and ABS pump, if the servo doesn't fix it it's going for an mot as is, if it fails on brakes it's getting scrapped!

[moodybluetwo]

Hi…I’ve had a similar situation to this twice…although not on a Peugeot. On each brake calliper there are 2 sliders that allow the brakes to adjust as the piston pushes….well if one of those sliders stick (and they do) the calliper will push out at an angle and the effect is the brake peddle travel increases. You will know which brake unit is faulty if you check the brake pads….the faulty callipers pads will be worn at an angle…not flat.

[jasper5]

As Doggy said in a previous post, pump your brake pedal until the servo is empty and the brake pedal is solid.

If the brake does not sink then you do not have a problem with the brakes as such.

You could only have a servo fault in this case.

As Steve said, sometimes this is perfectly normal, but I would need to drive your car to be able to tell.

[RobTurbo]

With the engine off the pedal goes rock hard, I don't know if that means it's a problem with the servo or if the extra force from the servo is making the problem occur. I spoke to the mot tester about it and he told me the last time he had experience of something similar was with a rover and was caused by a faulty ABS pump, but said it was much worse than my car is, just by description, he never tested it, it's booked in for the end of the month.

Tomorrow the callipers are getting a good overhaul and new pads which is the final step before I order a servo!

[lozz]

Have you checked the brake Compensator, ?
not sure if all the 4o6s have them tho,

[boombang]

With the engine off the pedal goes rock hard, I don't know if that means it's a problem with the servo or if the extra force from the servo is making the problem occur. I spoke to the mot tester about it and he told me the last time he had experience of something similar was with a rover and was caused by a faulty ABS pump, but said it was much worse than my car is, just by description, he never tested it, it's booked in for the end of the month.

Tomorrow the callipers are getting a good overhaul and new pads which is the final step before I order a servo!

I'm not that up on the 406 electrical system, but can you not disable the ABS by pulling the relevant fuse then give it a go?

[steve_earwig]

I'm not sure what the pedal does when the abs kicks in on the 406 (it so rarely happens these days ) but might this not be a result of the abs ecu being confused by a broken reluctor ring? Pull the fuse, as boombang says (or pull off the connector if you can't work out which fuse it is) and see if you still have the problem.

[RobTurbo]

The ABS kicks the pedal back up rather than letting it sink, and that's working fine. When I was changing pads and cleaning callipers yesterday I found the OSR calliper was missing the dust seal, the piston is stiff but does still move, but one of the sliders is seized solid, I can't see how this would cause a sinking pedal because it's not leaking, but needs sorting anyway so I've ordered a new calliper, I'll wait until I've got that fitted before I try anything else.