Sinking brake pedal, not master cylinder

[RobTurbo]

It's not so much the sinking pedal I'm bothered about, it's the delay from pressing the pedal to the car slowing down, it's as if there's no servo assistance until you hold the brake on for a while and it's definitely not normal!

[madmadmax]

that could be a vac leak then check all the vac pipes and the vac pump that will need a vac gauge

[RobTurbo]

Already swapped the vac pumps over between this 406 and the 306 I've been braking, the 306 was still fine and the 406 is still broke! Pipes are all in good order, no leaks or anything, I've changed the rubber seal where the vac pipe goes into the servo.

The only bits I haven't checked/changed are servo, abs pump/block/ecu (I have tried unplugging the ecu and driving without it and it was still exactly the same), and linkage from pedal to servo.

[RobTurbo]

New servo came today, got it fitted and it works a treat!, I now have good brakes!

I managed to change it without taking brake pipes off, it took a good bit of fiddling, especially the pin (mainly the clip on the pin) that connects the servo to the brake pedal link rod.

Haynes says you have to take the whole linkage off and even suggests removing the cylinder head to improve access

So with that problem out of the way I can get cracking with making it better!

[steve_earwig]

Thank feck for that! I know it had to be something but I wasn't sure if the servo could actually produce such a problem. They obviously changed since I last had one apart (like so many things )

[RobTurbo]

I pulled the old servo apart once I knew the brakes worked properly with the new one, but couldn't find anything obvious wrong with it, must be something to do with a valve or something, I'll strip it right down when I get chance.

[highlander]

sh*t

I have problems like this on mine, except mine's a petrol.

I had the brake hoses replaced with braided ones about a year ago, and some funny stuff started happening not long afterwards. I noticed the pedal would sink (even after the car stopped moving), but ruled out a servo problem by doing the "pump the pedal with the engine off" test. I also noticed a sort of "crunchy" feel while pushing the pedal, like I'm pushing my foot into gravel - and when I release the pedal after it's "crunched", I hear a faint hum for a second, like something motorized is moving. Worse still, sometimes I'd press the pedal and it would go down maybe an inch before going totally solid, during which time the car would not slow down Nearly rear-ended some poor sod in a Corsa as a result. So I decided to take it to a garage, and on the way over, the "brakes not moving" symptom kicked in, and I had to use the handbrake to stop the car (as it was busy trying to sail through a red light on to a busy crossing...)

So I just had the car into a garage and they changed the master cylinder and one of the ABS sensors (they also changed a faulty fuel injector but this was unrelated to the braking issues). They also said that there was something "bent" on the rear brakes (will need to check the service report when I get home to see what that something was ...) and they apparently straightened it out too. But when I picked it up, they said they could hear some kind of air rushing when they press the pedal, and decided that would likely be a fault with the brake servo. They didn't replace it, but told me I'd need to do it soon, and said the brakes felt perfect. Yet when I took the car home on Saturday, I found that the "sinking pedal" sensation had gone, but the "crunchy gravel" symptom was still there, damn near every time I applied the brakes. I haven't driven the car since I got it home on Saturday (I tried to phone them back but they'd closed by that point). I will be driving it again tonight, and if it happens again, I'll be phoning them up to complain, bitterly. They charged me £487.19 all-inclusive for the work they did, and I can't afford any more work being done - I really hope I don't have to replace any more parts, and it sounds like a brake servo might be expensive. It also took them more than a week to get the master cylinder; I'm guessing it'll take them ages to get the replacement servo too.

I now want to sell this car on (can't afford a replacement at the moment though) and I think most people would be put off buying it when they get the "crunchy" pedal symptom.

Is the "crunchy" pedal symptom likely to be as a result of the brake servo, or something else?

[lozz]

Re: "crunchy" pedal symptom

Have they bled the ABs system ?

[highlander]

Not a bloody clue

[bangernom1cs]

I'm going to do both a master cylinder and servo swap shortly.
My MOT man says that I only just cover the braking standard with mine and thats despite
changing hoses ,pipes and rear calipers. Along with a few sets of discs and greasing up all
the caliper pins so that they move freely. Only the servo to change as I'm still getting that delayed action under sudden braking.

T

[bangernom1cs]

Ok so today we took the servo off and BOY is it a fiddly job to do!
Best access is to go via the LH front wheel arch with the liners removed.
Also there is a small plastic splashguard on the bottom of the bracket which holds the servo to the main body so make sure you remove this also. The pin and clip behing the servo is very tricky indeed.
No evidence of fluid being lost even though I have pedal sink, so the by-pass theory in the ABS pump seems to be right,

The good news is that I now have instant ultra sharp brakes on the car ! And none of the delayed action that it had begun to suffer from And as a bonus, with full vacumm restored I seenm to have regained proper turbo action as I was experiencing flat spots and limp mode particularly in the high gears and trying to accelerate up hills. There was one spot up a hill on the A55 near Broughton that the car seemed to hate! So those suffering turbo lag and think its down to the booster action this might pinpoint a problem.

Will keep you updated.

[steve_earwig]

That lift - I want one!

Nicely done sir!! I'll bet that's a weight off of your mind

[rugmania]

Hi my 406HDI 03 has started to decide not to brake properly!! I have taken the car off the road because the brakes are faulty!! The servo is fine when braking normally, but if i brake quickly the servo forgets to work. Its rather alarming. I am not loosing any fluid, brake pedal goes slowly down to floor when idling,with engine off and pedal repressed a hard pedal at the near top. can anyone help to resolve this problem?

[GingerMagic]

Hi.
The brake servo shares it's vacuum supply with other engine components - so I guess a gentle press and there is enough vacuum to work, but a sudden vacuum request means there is a shortfall....
Check your vacuum pipes for split ends etc, and if you can access a vacation gauge then this will help identify any problem areas - hopefully it will be an easy fix.
It's not like you can rely on the handbrake to slow you down..!

[rugmania]

Thanks for the reply I will get the vacuum checked, and look at the other pipes leading somewhere?? Will let you know. Thanks