Pulsing Brake Pedal - Symptoms Listed - Any Ideas ?

[macspring]

S2 Avant driving fine, no issues, parked the the car, a daily driver, left it un-drvien for two days over the weekdnd, disconnected the battery after the second day as it has a "electrical drain" that flattens the abtttery after a couple of days, until I have time, it's easier to disconnect the battery if I'm not driving it for say two days or more.

When I subsequently reconnected the battery and drove the car, the brake pedal pulses when used, and feels very hard early on in the pedal travel. If one presses hard when idling it will descend, but takes a lot of effort.

Also the at the rear of the engine / engine bay the small, light blue in colour, control valve 034 133 517 (N80 - Evaporative Emissions Control Solenoid Valve) is pulsing rapidly, especially on overrun. It is normally quiet.

This has pulsing of the pedal and the blue valve has happened to me before with this car when it was stood for long periods of time, and in the past when one continued to drive it out of necessity, it sudden goes away after a say 10 days days / a few weeks, and the brakes work as per normal.

Wanted to know what it is, to be able to speed up the process to get it back to normal.

The power steering pump is fresh, enough and the bomb is not ancient. Hnadbarke working fine at speed, it not the disks, or the calipers.

Annoying thing is it was fine when I parked it and two days later, it is not, with the only change being the disconnection of the battery for the second day.

Just realised now the other thing I did was power wash the car, and as it hadn't been washed for months, especially under the arches, and so I sprayed the water in there to get the mud and the salt off the plastic wheel arches etc.. I'm now thinking I need to dry out the ABS connections under the four respective arches ?

Any thoughts, or have I just confused everyone ?

Don't believe it is the brake servo, as it has cured itself before, but for just for information, does anyone happen to have the correct ATE part number for an S2 ABY Avant master cylinder, and it's approximate cost ?

Thanks for any assistance.

[5cylinderman]

I had a similar experience on a '00 Passat a few years back. Somehow one of the ABS sensors had moved in the hub and the air gap between the sensor and the pick-up ring had become too big. The pulsing was the ABS kicking in, as opposed to the pulses you feel at the pedal when your PAS pump is either low on fluid or on its last legs.

[popdemonic]

Pedal hard and pulsing things mean you had no power assisted
on the hydro system.you may try to pull off the oil screen line and
Clean them.if one is clogged,it will also do that.when i converted
from vacuum back to hydro.i was surprised when there was
no assisted from the system.replaced the bomb also didnt help.
Had them cleaned with carb cleaner.it was reincanated from the death.

Brake servo failures,it just shows you the (!) after
You have parked a car for short or long period on first
start but the pas pump will be pumping and getting enough pressure
in the system afterward.this's just annoying us everytime
when start up the car.

One thing,does it has the (!) on your instrument cluster?
And make sure you have proper wires connected to the pressure
Sensor on the servo.if there's no (!),the above statements are useless
for you.it's something another story.

[macspring]

Sorry, yes, it is displaying the (!) in red on the dash. Fluid level is fine.

Thanks.

[Rusty]

Brake servo is on its way out. Fluid flush or get a replacement. I had this and fixed it last year.

[here2learn]

i had this issue when i put my engine back for the first time i changed the pas belt and happy days not always a simple fix. but best place to start

[Error404]

Brake servo is on its way out. Fluid flush or get a replacement. I had this and fixed it last year.

Servo seals are worn and its taking ages to pressurise.

[popdemonic]

I have 5 brake servos.All of them can be blown by more or less.I think since its age caused this.consequently,it discharged the pressure gradually when you just shut down the car.the more it blow by,the faster it discharged the reserved pressure from the bomb.I think the brake servo is proned to be more often failed down than bomb today.i ever disassembled and found kinda teflon seals or something.i don't think it's replaceable since it needs the heat/thermal to shrink the seal down to fit the shaft/piston inside the servo.if you're tired of this hydro system,get the vacuum booster.it's good for
DD but i don't like deadly solid pedal feels.only way i'd convert it back to vacuum system again is the dual diaphragm type from D2 A8.I found a local C4
car converted it to dual diaphragm booster but i'm just wondering how mechanic done this since the C4 uses 4 bolts mounting pattern booster on the firewall while the D2 A8 uses 2bolts pattern(Diagonal) mounting on the firewall.Maybe it has the same bolting distance and just mounting it on the firewall with only 2 bolts.If anyone knows the bolts distance on the D2 A8 booster,then the rest of part is piece of cake.

[macspring]

Update,

Changed the master cylinder / servo, i.e. the thing that the brake fluid resovoir "plugs" into that the two "copper" brake lines comes out of, with one from my "spare" S2 Avant. This made a marginal improvement. It also meant I had to bleed the brakes and so flushed through all of the old stale brake fluid.

Funny thing was I found tha the clutch was slghtly better in that , since putting the car back on the road after a year, with an uprated clutch I had found that getting the car off the line in normal city driving was difficult and jerky. I had put it down to driving the auto urS6 for a year, and the high biting point of the cluch because of the ABY with a 3b flywheel and no gearbox spacer.

But with the newer fluid, it is smoother and easier to get off the line without so much of a bogging down jerk.

Might be worth people bleeding their clutch line.

So back to the origianl issue of the brakes, I swopped out the known to be working bomb from the urS6 and put it is the S2 Avant, it felt a bit better, but still didn't fix it, but drove the car today anyway as it is a dd. Earlier this afternoon, the system "bled" itself / bedded in and the brakes are now perfect again, as the factory intended, with no pulsing on the pedal, no clicking from under the bonnet, and normal pedal travel and feel / pressure.

So most likely the whole thing was caused by the bomb needing to be replaced.

Was ready to swop in the working master cylinder / servo from the urS6 into the S2, but no need now.

btw, the urS6 came with the bigger oem "skeletal" calipers for 314mm discs, (forgotten their proper "name") so does this master cylinder have bigger capacity than an S2 one, and so might it be more suitably matched for 323mm GT3 calipers and 323mm discs, thereby giving less close to the floor brake pedal travel, or am I just letting my mind wander too much, and they are both the same, with no advantage to the urS6 brake master cylinder?

I presume there is no way of checking a bomb, other than to fit it to a car and try it out for a few days ?

Why are my posts always so long ?

[mcandmar]

As above when the bomb loses pressure the pedal pulses, what your actually feeling is the hyd pump working. As Rusty says worn seals in the Brake servo are the main culprit allowing fluid to bleed out when the cars sitting, or if its really bad it will prevent the bomb from building any pressure at all and you will have no brake assistance.

Fair play for swapping it all out though, must have been great fun skinning knuckles with fluid running down your arm

[macspring]

Fair play for swapping it all out though, must have been great fun skinning knuckles with fluid running down your arm

Knuckles intact, but arms and back soaked in hydraulic fluid.

Couldn't find your phone number ! lol.

Just need to swop out the failed rear boot handle, and cure the wot 4k strangulation issue this weekend and it'll be perfect for Tuesday.

Oh then swop back to oem exhaust , and loosen all of the engine and gearbox mounts to settle / change / fettle the mounting of the engine and gearbox to see if I can improve the fitment of the 034 3"inch downpipe which is tight on the subframe and also the metal CV heat protection shield.

At this stage with hindsight, I feel I should have stayed with the RS2 downpipe and RS2 exhaust system, much less hassle, for not much of a trade off in performance.

Hope you're learning from my mistakes.

[irishs2]

Glad you got it sorted. I hate the bomb.

The urS6 has the same master cylinder as the S2, 25.4mm .