Introduction | Fundamentals | Pads | Discs/Rotors | Big Brake Kits

Brakes & Upgrades - Fundamentals


Hydraulic Configuration

The hydraulic configuration of the S2 is that of a traditional twin circuit master cylinder design. However, instead of the brake circuits being connected to diagonally opposite corners of the car, as on the vast majority of FWD or RWD cars, the circuits are actually split front and rear on account of the 4WD transmission. This is a common trait on all Torsen equipped Audi quattros as far as I know.

The exact technical reasoning behind this has always eluded me because it is well known that drive to the rear of any Torsen equipped quattro is effectively disconnected when in liftoff/overrun/braking situations. Of course, the situation would have been different in the earliest non-Torsen UR quattros so perhaps Audi just decided they liked the front/rear split and stuck with it.

Needless to say, S2 owners won't complain as a front/rear brake split is intrinsically more stable and easier to balance than a diagonal split. For those of us that occasionally left foot brake, there is added confidence in having that front/rear split as the brakes can be applied whilst power IS transmitted to all four wheels.

Unlike the UR quattro, the location of the master cylinder and servo is dependent on whether the car is left or right hand drive. This is a significant improvement for RHD S2's as the connection from the brake pedal to the master cylinder is significantly less complicated than RHD UR's . The servo assistance on LHD cars is 3.8:1 whlist it is 4.7:1 on RHD cars. The lower servo assistance ratio on LHD cars is presumably because the ABS equipment always resides on the left side of the car.

All S2's were equipped with ABS as standard equipment - regardless of LHD or RHD, the ABS was fitted to the left side of the engine bay. Click here for more ABS info.


The brakes receive power assistance from an ingenious hydraulic servo device which is pressurised by the hydraulic pump for the power steering. The reason for this is twofold. Primarily that it is a more compact package (squeezed between the brake pedal and master cylinder) than a traditional vacuum servo. Secondly, the complications of ensuring vacuum is available on a car which spends a good deal of time creating boost in the inlet manifold are equally cumbersome. The S2 is not unique in its use of a hydraulic servo as it was used on the URs and the 7A engined 20V CQ to name some other applications. Click here for more info on the hydraulic servo system and the brake pressure accumulator which is it's most infamous component.
DOT4 is the only brake fluid to be used in the S2 and it must be renewed at least every two years.


Master Cylinder

A major consideration in the braking potential of any vehicle is the capacity of the master cylinder.

The supreme brakes of the Porsche/Audi designed RS2 Avant are an excellent benchmark for how good the brakes on the S2 could be.

It is therefore very reassuring to know that the master cylinder and servo used in the RS2 is exactly the same as that used in both the 3B and ABY incarnations of the S2.

Click here for some more introductory information about the brake pressure accumulator (also known as 'The Bomb').


Brake Pressure Regulator

A non adjustable brake pressure regulator is fitted to the S2 in a rather unusual place - alongside the electro-hydraulic unit belonging to the ABS.

The function of the brake pressure regulator is to reduce the amount of hydraulic pressure that is applied to the rear brakes. This ensures the rear brakes do not lock up prematurely under heavy braking as load over the rear wheels is greatly reduced. Furthermore, failure of the front brake circuit activates a stop-piston which enables full brake pressure to be passed to the rear.

To check the function of the brake pressure regulator, specialised brake pressure guages (VAG1310) need to be fitted to the bleed screw points on the calipers. The correct specification on front and rear brake pressure is dependent on model year and is shown in the table below.

For cars built from MY92, (chassis PA 000 001 onwards) there is a different brake pressure regulator that is marked with a blue dot and the number '35'.

Note that higher rear brake pressure are provided on the later system. My unproven theory on this, is that Audi figured that some of its problems with rear caliper seizures across the earlier 80 series was because the rear calipers endured such little work and so would be prone to seizure.


Front pressure
(bar)
Rear Pressure
Up to MY92 (bar)
Rear Pressure
From MY92 (bar)

50
30 - 35
37.5 - 41.5
100
45 - 50
51.5 - 57.5


Brake Hoses

As with any car, a number of flexible brake hoses are needed on the S2, to provide for suspension and steering movement between the brake caliper and the bodywork. Because these hoses aren't as rigid as the metallic brake lines they can reduce pedal feel.

There are actually six such hoses on the S2 Coupe, with two being used at each front wheel and a single one at the rears. These can all be replaced with stainless-steel braided hoses available from specialist suppliers for a slightly stiffer pedal as they do not expand under pressure to the same degree as normal flexible hoses. Details to follow...
Brake Bleeding Procedure

The basic rule (excluding the clutch) for bleeding the brakes on the S2 is to bleed the caliper which is furthest from the master cylinder first and then move around the car finishing up with the caliper nearest the master. That means we have a global brake bleeding order -
1. Clutch (Master then Slave)
2. Master cylinder (if bleed screw fitted)
3. Rear Passenger Side

4. Rear Driver Side
5. Front Passenger Side
6. Front Driver Side
For what it is worth - I have never encountered an S2 with a bleed screw on the master cylinder.

NOTE - Always make sure the handbrake is OFF before attempting to bleed the rear brakes. Failure to do so will not bleed them properly. Ask me why I know this !

When changing the brake fluid it is advisable to bleed at least 250ml of brake fluid through each bleed screw. The official factory workshop manual actually recommends that 500ml of fluid is bled through each caliper. This can be reduced if the system is drained of old fluid beforehand. A good pressure bleeder kit is a worth-while purchase as it makes for a simple one-man job.

Brake Fluid

A major contributor to brake problems on any car is the quality of brake fluid. Any S2 owner will tell you that they would like brakes with superior resistance to overheating and fading that results from it. Even with the biggest discs and calipers that money can buy, the brake system will be of little use if it allows the temperature of the brake fluid (in the calipers) to reach it's boiling point. Such an event results in a soft pedal that can be unsettling or outright dangerous depending on your luck...

Therefore, a very worthwhile upgrade to the S2 brakes is to use the best brake fluid you can afford.

The table below lists a number of aftermarket brake fluids which all meet and exceed the requirements of DOT4 specifications. Compare these to the minimum spec for any budget brand DOT4 fluid and you will start to understand why decent brake fluid is so important. Needless to say, the price goes up at the top of this table.

Brake Fluid
Dry
Boiling Point (F)

Wet
Boiling Point (F)

Castrol SRF
590
518
Motul RBF600
593
420
AP 600
572
410
ATE Super
536
392
DOT4 Minimum
446
311

The most important column in the table above is the 'wet boiling point' which reflects the performance of the brake fluid when it has absorbed a specific %age of water which naturally occurs over time. All DOT4 fluids absorb water to varying degrees and this is why fluid changeovers are necessary at least every two years.

A neat feature about the ATE Super brake fluid is that it is available in two colours (Blue and Gold) to make fluid changeovers easier.

Remember - Spilt DOT4 brake fluid will damage paintwork.

Finally - NEVER ever use DOT5 fluid on the S2 as it can cause internal problems with the ABS.

Standard Components - Front

These twin pot calipers made by Lucas / Girling have the designation 'G60' and were a common fitment to the Audi 80/90/100/200 family. The same components were also fitted to the 20V UR quattro. The part numbers for the front calipers are as follows -

Front Caliper Parts
Part Number
Front Caliper (Left)
895 615 123 A
Front Caliper (Right)
895 615 124 A
Front Caliper/Pad Carrier
447 615 125

The asymmetrical piston diameters of 36mm and 44mm are oriented such that the smaller piston works on the leading edge of the forward rotating brake disc. This provides for a progressive pedal feel and is intended to reduce disc/rotor vibration.

The front calipers are mounted to the trailing edge of the hub carrier on the S2. The same calipers, fitted to other Audis are sometimes on the leading edge so be careful not to confuse matters if sourcing similar looking brake parts from other vehicles.

The dimensions of the standard ventilated discs on the S2 are also no bigger than the lesser powered members of the Audi 80/90 family. Disc details are as follows -

Front Disc Part Number
895 615 301 D
Front Disc Diameter
276 mm
Front Disc Thickness (new)
25 mm
Wear Limit
23 mm
'Hat' Height
56 mm
Hub centre diameter
68 mm
Wheel nut PCD (5-bolts)
112mm


Front Brake Maintenance


Changing the front pads on the S2 is a cinch for the experienced DIY mechanic. Obviously the usual disclaimers apply about qualified people working on brakes and using the correct parts at all times to ensure maximum safety. If in doubt, get it done professionally !

To remove the front caliper and access the pads, a 13mm socket or spanner is needed whilst holding the retaining nuts with an open-ended 17mm spanner. Never allow the caliper to dangle by the brake line - support it in some way so as not to damage the hose. When pushing the piston back into the caliper, keep a close eye on the brake fluid level in the reservoir so it does not overflow.

The caliper retaining nuts are actually lubricated guide pins which slide into the caliper carrier. These guide pins can seize over time if the dust seals get damaged and dirt is allowed to penetrate. This can cause uneven braking which eventually leads to vibration and premature disc wear. Always check the condition of the guide pins and dust seals when changing pads. A kit comprising two dust seals, two mounting bolts and guide pin grease is available for one caliper as part number 443 698 470. Note that guide pins are NOT available from Audi as a replacement part.

New pads from Audi normally come with a set of four caliper mounting bolts. These are pre-coated with a thread-locking compound. If buying after market pads then don't expect to get the bolts included. It is wise to renew the caliper bolts (321 615 141 ) at every pad change.

Replacing the front discs is a little more problematic on the S2 as it requires the tightly mounted caliper carrier bracket to be removed. These can put up a good fight. The two 19mm headed M12 bolts which hold the front caliper carriers into the hub are subject to corrosion so one needs to be very careful in removing these bolts. The carrier mounting bolts ( N 901 740 01) should of course be renewed when the discs are replaced.


Torque Settings


Caliper self locking bolts - 35Nm
Caliper carrier bolts - 125Nm
 


Standard Components - Rear

The single pot rear calipers are also made by Lucas / Girling and were a common fitment to the rear of the Audi 80/90/100/200, many VWs and a host of other European built cars. The part numbers for the rear calipers are as follows -

Rear Caliper Parts, Coupe
Part Number
Rear Caliper (Left)
853 615 423 A
Rear Caliper (Right)
853 615 424 A
Rear Caliper/Pad Carrier
443 615 425 A
Caliper Mounting Bolt
N019 912 3

The rear brake calipers on the Avant & Sedan models are slightly different to the Coupe. This is because of the different routing for the handbrake cable - caused by the differences in rear suspension between the Coupe and the Avant/Sedan layout. More about that to come in the Suspension chapter. Its also worth noting that the Avant/Sedan rear caliper uses two different sized bolts which bolt the caliper carrier to the hub carrier.

Rear Caliper Parts, Avant/Sedan
Part Number
Rear Caliper (Left)
8A0 615 423 A
Rear Caliper (Right)
8A0 615 424 A
Rear Caliper/Pad Carrier
443 615 425 A
Caliper Mounting Bolt (Short)
N101 815 02
Caliper Mounting Bolt (Long)
N102 409 01


In both cases, the rear calipers use a single piston of 38mm diameter. You can clearly see the number 38 cast into the caliper body. The handbrake mechanism uses a spring loaded cable assembly which operates on an actuator to press the piston towards the disc/rotor. This mechanism is famously problematic on all the VW/Audi applications and proper maintenance is needed to prevent the rear calipers from seizing. More about that here .

The dimensions of the standard solid discs on the S2 are also no bigger than the lesser powered members of the Audi 80/90 family. It is particularly frustrating when you know that much bigger discs were fitted to the rear of the 20V UR, the 200TQ Avant and the UrS4/S6 but none of these can be bolted straight onto the rear of the S2. More about that elsewhere... Disc details are as follows -

Rear Disc Part Number
895 615 601 A
Rear Disc Diameter
245mm
Rear Disc Thickness (new)
10mm
Wear Limit
8mm
'Hat' Height
40 mm
Hub centre diameter
68 mm
Wheel nut PCD (5-bolts)
112mm

Rear Brake Maintenance

Changing the rear pads on the S2 is also fairly straight-forward but the use of a special piston wind-back tool is highly recommended.

Please note the pistons in the rear calipers do NOT push back - they must be carefully rotated back. This can be done with care using fat blades screwdrivers or other home grown devices, but the wind-back tool (common to all Lucas/Girling calipers of this type) makes the job a doddle.

The same disclaimers apply about qualified people working on brakes and using the correct parts at all times to ensure maximum safety applies. If in doubt, get it done professionally !

To remove the rear caliper and access the pads, a 13mm socket or spanner is needed whilst holding the retaining nuts with an open-ended 15mm spanner. Never allow the caliper to dangle by the brake line - support it in some way so as not to damage the hose.

To wind the automatically adjusting piston back into the caliper, so as the new pads will fit, a special tool is required. This is VAG3272 and it is mighty expensive from Audi. Fortunately suitable alternatives are made by Sykes Pickavant and Laser Tools in the UK as the rear calipers on the Audi 80 & S2 are shared with many other cars. The wind-back tool features a reaction plate which ensures the piston is quickly and easily wound back. With luck and care, it is possible to wind back the rear piston with long nosed pliers but it takes ages and the risk of tearing the piston dust seal is very real.

A similar arrangement of lubricated guide pins and dust seals is used on the rear calipers. As with the front brakes, the condition of the pins and seals must be checked when new pads are fitted. The part number for a one-caliper kit containing two dust seals, two bolts and grease is part numbe
r 443 698 470 .

New rear pads from Audi also come with a  set of coated caliper bolts (4-off 321 615 141). These are the same bolts included in the dust seal kit.

Replacing the rear discs also requires removal of the caliper carrier. Care is needed with these rather tight bolts and the use of a good quality 8mm hex/allen 'cheese head' socket is required to despatch the mounting bolts with the minimum of fuss. These bolts should also be renewed along with the discs.

Torque Settings

Caliper self locking bolts - 35Nm
Caliper carrier bolts - 80Nm

Handbrake Adjustment

Before adjusting the handbrake, firstly check that the handbrake cable is not pretensioned with the lever in the off position. To do this, ensure that the handbrake cable levers (on top of the rear calipers) are on the stoppers at both sides of the car. A large screwdriver can be used to check if the handbrake mechanism is moving on the calipers.

To adjust the handbrake (normally only required after cables, calipers or discs and pads have been replaced) there is a threaded rod with a nut that moves the compensator bar.

A 10mm ring spanner is best for the job. You may need to remove a heat-shield near the exhaust to access the adjustment nut. Increase the cable tension by tightening the 10mm nut such that BOTH handbrake levers just lift off their stop on the calipers.

Then back off the adjusting nut one or two turns and check that both rear wheels turn freely.

Set the handbrake to the ON position inside the car (three or four 'clicks' according to taste) and check that the rear wheels are firmly locked.


Rear Caliper Problems

It has been suggested that there are two main reasons for rear caliper seizure, and the accompanying handbrake failure, on the Audi S2 - and the tens of thousands of other cars fitted with the same system.

The first issue is the combination of dirt and moisture which collects around the handbrake mechanism at the rear calipers. When servicing the rear calipers (or fitting new ones) it is wise to carefully dollop some waterproof grease around the handbrake actuator to prevent moisture ingress to the mechanism.

The second reason is that the return spring for the handbrake actuator on the S2 rear caliper isn't quite strong enough to ensure proper movement of the mechanism.

It is possible to fit a stiffer return spring onto the S2 rear calipers for improved operation of the handbrake and much increased resilience to caliper seizure. The simple procedure involves sourcing the springs fitted to similar calipers from a Ford Granada (1990 approx). New springs can also be obtained from your local Ford parts counter, but they can be difficult to source nowadays. The Ford part numbers for these springs are 6141147 & 6141148.

Credits to Ben Buchanan for providing this information from the succesful modification he made to the rear brakes on his 1991 S2 Coupe. Thanks also to Phil Payne for the Ford part numbers .

Brake Pressure Accumulator - The 'Bomb'

This ingenious component, not unique to the S2, gets it's nickname from the crude physical similarities with one of those comedic explosive devices as featured in Monty Python, The Pink Panther and Road Runner to name a few.

The purpose of the brake pressure accumulator is to provide a reserve of sufficient servo assistance in an emergency brake situation. In normal driving, the power steering pump provides servo assistance, but in emergency braking, the pump provides little assistance to the servo. A worse scenario is the engine stalling under heavy braking. The brake pressure accumulator is designed to provide a reserve of highly pressurised hydraulic fluid for situations when the power steering pump is unable to create sufficient pressure. The 'bomb' is charged with Nitrogen gas under high pressure to independently ensure that brake servo assistance is always available. For further description of the brake pressure accumulator, and how to test it and the power steering pump - follow this link .

Important Note - The ONLY hydraulic fluid which should be used in the S2, is Audi's G002000 - a green coloured mineral based oil. Use of anything else can cause major damage to the hydraulic system.


On a car with a healthy 'bomb', and the engine not running, the brake pedal will require more than thirty pumps to fully discharge the stored brake servo assistance. This results in a 'dead' pedal feel when the servo is empty.

Over time, the 'bomb' can leak - allowing the Nitrogen gas to escape resulting in a loss of performance. When this happens, fewer pumps of the brake pedal will result in the brake servo being emptied (as less and less pressure is reserved in the bomb). When the number of pumps gets below twenty then think seriously about replacing the bomb... and when it gets below ten pumps, the bomb is very close to the end of its useful life.

Eventually, when the bomb is well and truly past it's best, a pressure sensitive switch in the brake servo will activate a red warning on the Autocheck system alerting the driver of a brake problem. Such an alert will often clear when the engine is started and servo pressure is temporarily restored by the power steering pump. If the pump is not providing adequate pressure then the red hydraulic system alert can also be activated. For more diagnostic details on the bomb, click here...




Last Updated 29th June 2007