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.
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.
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.
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.
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.
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.
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.
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.