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Scenic servo hissing when brake pedal released - fixed??

45K views 16 replies 7 participants last post by  Lagdti 
#1 · (Edited)
Car in question is a Scenic 1.4 16v MK1 phase 2, 2002 manual box with abs.

This post refers to a temporarily permanant??? solution to the brake servo hissing when the brake pedal is released. If your braking performance is still fine and you just have the hissing then this may suit you. Any other servo problem such as a hiss when pedal pressed will probably need a new servo.
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Forum disclaimer.
Carrying out this repair was my own personal choice and is by no means endorsed by the the owners of this forum or it's associates.
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It involves a slightly unorthodox repair to a brake component but has been tested by myself over many months with no ill effect to the braking system. If I thought there was a chance of brake failure occuring afterwards, I would not have taken the risk.

Why is it hissing?
If you look up the back of the brake pedal you will see a ribbed rubber boot. This is a dust cover for the tube that slides into the servo when you press the pedal.
At the opening to the servo is a rubber seal. The tube on my servo was fairly dry and as the tube comes back out, it tries to pull out and distort the seal giving an air leak. The rubber boot also clips over the edge of the seal and also pulls it slightly.
As the seal moves, it tears off bits of it's outer retaining ribs and they can cause distortion and may make the seal grip the tube more which again, will cause the seal to be pulled out.
My hiss started intermittantly but got worse over a few months.
Auto part Automotive exterior Vehicle Suspension
Auto part Machine tool Machine Wheel


Access to the boot and seal is poor so I removed the pedal box and left the servo and master cylinder attached to car. They will be hanging on the brake pipes but they are more than capable of supporting it as long as you don't start forcing things. There will be no need to disturb any of the brake fluid circuit or the seal between the master cylinder and servo by doing it this way.
The servo can be removed but you need to remove 2 nuts from the engine side which hold the master cylinder to the servo. Access is not good.
On a scenic the servo can come into the car with the pedal box- supposedly.

What tools?
Good work light, screwdrivers, 3/8 drive socket set, Torx bit set, spanners, torque wrench.
You will also need small a tube of silicon sealant.

You will need to allow time for the sealant to fully dry so it might be wise to leave the car overnight before reassembling it and driving.

At last- how to do it.
It is also covered in your Haynes book if you need it.
A couple of the pictures are of a different servo which I have just incase they have you wondering.

Auto part Automobile pedal Automotive engine part

It was a while ago but here goes.

Disconnect accelerator cable and remove pedal.

Fold carpet down behind itself.
Remove the sound deadening held by the black plastic discs. Also 1 or 2 high up over the top.
I did unclip the fusebox to look through the hole to see what I was doing but it may not be necessary. Just saves straining your neck a bit.
Shove the sound deadening up behind the dash if it won't come out easy.

Disconnect clutch cable from the gearbox- do not move the arm once cable is off.
Disconnect cable from pedal noting it's route first.

Carefully remove connector to brake light switch.
Remove clip holding servo to brake pedal by lifting the end and sliding clip off.


I removed both pedals at this stage by removing the pivot bolt and bracket on left hand side.
Make a drawing of the order of parts that the bolt runs through.
Most of the plastic bushes stay attached to the pedals and the clutch ratchet is riveted in place so bits don't fly everywhere. There are a couple of metal washers that will drop out so watch for losing them. Don'r forget the spacer tube inside the brake pedal on refitting.
Watch you dont lose the small strip of rubber that the clutch pedal hits once it's up.


Remove 4 nuts holding servo to the mounting plate.
Remove any bolts holding mounting plate to body.
Pull mounting plate away from body slightly.
Push the 2 tags on the clutch cable where it comes through the plate and push cable out.
Do similar with accelerator cable but mark which hole it's in first to save confusion later.

The plate should now come towards you but it's a fair wiggle job to get it out.
Try not to dislodge the brake switch.
If the plate hits the wiring loom running across bottom of dash, unclip it and push it up out of the way.
Eventually it should come out.
Auto part Brake Automotive exterior Motorcycle accessories Disc brake
Auto part Brake Vehicle brake


Pull back the boot on the servo to reveal the seal.
DO NOT wipe off any oily goo from the white tube. That's lubricant for the seal. Try and smear some onto a piece of tinfoil to use later.
I pulled the seal out and quite a bit of stringy rubber came with it.
Rip these loose bits off the seal but leave any that is still attached properly as these help keep the seal in place.
I then vacuumed around the seal gap on the servo to suck out any remaining bits of rubber.

You can now refit the seal but don't push it all the way in yet. Leave a few mm sticking out.

It's a tight squeeze but to help refit the seal you can move the tube sideways a bit to allow the seal past. Once you get the seal started you should be able to centralise the tube and push the seal most way in with your fingers. Don't damage it by prodding with a screwdriver.

Now take your silicon sealer and smear some under the lip making sure it's covered all the way round.
Push the seal fully home and smear any excess sealer around the seal.
Add as much as you need so you get it like in the picture. The idea is to seal it and also glue the seal into place without getting any in the servo itself.
Centralise the tube.
Auto part Machine Machine tool Suspension part


Lubricate the white tube close to the seal with the goo on your foil. If you don't have much of it, use a little fresh, clean engine oil.

You now need to wait for the sealer to dry before refitting the boot over the seal.
If you refit the mounting plate to the servo, I don't think you'll have room to get the boot on.
Don't run the engine until the sealer has fully dried or it will suck some into the servo and spoil the job.

After an overnight wait I fired mine up to make sure it wasn't still hissing before assembling everything, just in case.

When sealer is dry, refit the boot and reassemble in reverse but don't forget to pull the accelerator cable and clutch cable through the plate.
If you can't get the clutch cable to come through and clip fully into place, it should come through fully once you press the clutch when all parts are refitted.
Make sure the flat rubber gasket is in place on the servo before fitting plate fully in place.

Once everything is assembled, fire up the engine and try the brakes before setting off.

There is the possibility that a kit with the seal and white fluffy filter is available from dealers for some makes of servo but I didn't check further into this.
Finding a used servo that is guarantted to fit AND work is a struggle due to the amount of different part numbers out there. Not cheap for used or new ones. The choice is yours.

Torque settings courtesy of haynes-
Servo to mounting plate 20Nm or 15 lbf ft
Master cylinder to servo 23Nm or 17 lbf ft
Couldn't find a setting for the pedal pivot bolt and nut.

This was a mk1 phase2 scenic.
Is it exactly the same job on a Megane hatch or coupe? Sorry, don't know for sure.
Is it same on a mk1 phase 1?
Don't know. Look at the pictures and under your dash to compare.

If you have the tools and sealer then it's a freebie fix which can't be bad.
Yes, I know it was the most insanely boring post ever and will probably take longer to read than doing the actual job. Sorry about that.
Hope it helps get someone out of a hole sometime.
 
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#4 ·
Hi Dave, thanks a lot for sharing this, I'm thinking about attempting the fix myself as the problem appeared about a month ago and it's gradually getting worse. Until stumbling upon your post I had resigned myself to having to change the whole servo, quite a bit of money. A couple of quick questions, if you don't mind:
- what kind of silicone sealer did you use, if you still remember?
- was it hard to get the pedal retaining clip out and back on? Should I worry about ordering a new one beforehand?

All the best
 
#5 ·
Any kind of silicone sealer that dries to a firm rubbery cosistency will do.
B&Q decorating stuff is ok if you have any around.
It's only to seal arond the outer body of the seal and hold it in place.
I used Loctite blue gasket silicone for cars.

If you mean the clip on the rod that goes through the brake pedal, no it's easy to get off/on.
Put a screwdriver through it and twist to open it up a little.. Now just slide it over and off the pin. Should be no need to get a new one.

An after thought to this is that it may not be neccessary to fully remove the seal.
Just pulling it back a few mm to get some sealer in the gap may be enough.
The metal housing that holds the seal will rip off some of the strips of rubber as you pull the seal out, so in effect leaving it in situ may be better.
The seal will have been moving in/out slightly as you have been pressing the brake but won't have been much.

Write out the method in short hand version and take a look at all the parts you can see to familiarise yourself with the job before you do it.
Will make it easier.
Just refer back to the full boring version if you get stuck.

Go for it. I've probably made it sound worse than it really is.
:d:d
 
#7 ·
Thank you very much for posting this, I've spent most of the day sorting the seal on my grand espace 3.0 DCI. It's a time consuming and tedious job on the V6 as access is very bad. I couldn't actually get the servo off so I got it off and turned it around and did the job in situ. My seal had completely come off and was actually in the dust seal boot! It's all back together, but I need to bleed her now, any tips?

Cheers Mud
 
#9 ·
Well I finally found some time to give it a try. It all went smoothly up to the point when I had to pull the mounting plate in. On my car (2002 Megane 1 ph 2 sedan, which, I believe, was only sold in Eastern Europe so you might not be too familiar with it) the mounting plate is bolted on the outside of the firewall, between the servo and the body. I've attached a picture I took after I had the pedal box removed.
As I was unable to get enough access to the gasket in order to seal it, I just smeared some graphite grease on the plastic piston (it was completely dry), put the boot back, and bolted everything back in place. So far it does make a difference, it only hisses for about a second when I turn the engine on, then it goes quiet. I assume it does that because, when vacuum is applied, the gasket is now able to slide back in its place enough to do its job, but it remains to be seen how well it will hold in the long term.
 

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#10 ·
That's completely different as you say.
Looks like the side plates are spot welded and bolted so you can't get them off.:(
The rubber boot clips over the seal and will pull on the seal a bit as the pedal is lifted.
Even a few dabs of sealant around the edges of the seal might help.
Depends if you can still get anywhere near it now it's reassembled.
 
#11 ·
:crazy: - that's exactly how I felt when I realized I can't remove the side plates after unscrewing the four servo bolts. It took me a while and some profanity to accept there must be a reason for those things to be held in place by more than the four bolts :d

Anyway, to get back to the main story, I have to change the brake fluid soon, and, since I'll need to do some bleeding anyway I was thinking of completely removing the servo as well and fixing it outside the car. I'll see how it goes and let you know
 
#12 ·
**BUMP**

Sorry it's such a delayed response but whilst driving home about three miles from a friends house this evening, I too have noticed a hiss on my Megane 2 GT DCi 150. Is this the same thing? May need to take a photo but I'm incredibly worried now seen as I have also noticed fluid on the heel pad of the drivers footwell!
 
#14 ·
hi dave,

great write up anyways mines is suffering real bad with the whistle and pressure on brakes. do you know off the top of your head if i need to access the engine area to unbolt the mounting plate? mines in a diesel rx4 :/

im going to attempt this via your description but can you recommend a silicon sealer? this might be a stupid question but is a window silicon sealer ok to use? if not, then i will source one from fleabay or similar.

as for smearing the silicon on the outside of the ring is it safe to plaster the whole thing with it as your picture doesnt look to have alot of the blue sealant around the black tube.

also how long should this job take not including the time to let it dry etc...

thanks cammy
 
#15 ·
hi dave,

this might be a daft question but when you say

Disconnect clutch cable from the gearbox- do not move the arm once cable is off.

is there a cable attached to the pedal behind the image that you posted.

i dont understand where the cable is connected to the gearbox is this inside or outside the car and when you say do not move the arm once cable is off, what do you mean by the arm? and is it shown in image? if not, i cant take some photos and upload them should anyone need it for reference going forward.

cheers

cammy
 
#16 ·
ok i managed to get most of it off until i had to put it back together as im a little confused with the clutch cable side of things.

i see the plastic rod thats attached to the clutch pedal but maybe im blind but i cant see the clutch cable on the pedal? how does the plastic cylinder thing come apart from the plate or am i doing something wrong.

it looks like my seal has totally came out as it was lodged inside the grey plastic boot i did manage to get some of it back in but did notice a little part of the rubber seal o ring had broken (about a mm thick) so i tore it off probably about a third of it. anyways i can only get so much of the seal back in as it seems to be overstretched or its due to part of the o ring seal still being loose inside the main rubber seal.

if i can get a more detailed 'how to' remove the clutch cable and the dont move arm just incase i do something stupid i would greatly appreciate it.

cheers

cammy
 
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