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Audi A4 B5 Replacing the Clutch Master/Slave Cylinders

Replacing a defective Clutch Master and Slave Cylinders in a 1998 A4 Quattro (2.8 AHA)

This article will tell you how I replaced the clutch master and slave cylinders on my car. Neither I nor AudiWorld assume any responsibility for accuracy or applicability to your situation. Read these instructions carefully and take a close look at an Audi repair manual before attempting this job on your own car. Be sure to get proper torque figures and use a torque wrench to tighten nuts and bolts. The parts you are about to replace are plastic and will deform if not installed with some care. Follow all appropriate safety practices and remember that brake fluid is poisonous and should be disposed of appropriately.

With 99,000 miles on my '98 A4, in very hot weather (>100 degrees F.), occasionally my clutch pedal would not return after being depressed. I was always able to pull the pedal back up with the tip of my foot, but this was becoming more frequent and happened at bad times. It was not immediately clear if the cause of the problem was the master or the slave, but the price of the parts suggested that both could be replaced without breaking the bank. Both parts together came to about $160 and were delivered to my door without fuss from one of the many parts houses you can now find on-line. (I used, not the cheapest, but they include free shipping.)

This job will require:

  • Metric socket set
  • 6mm ball head Allen wrench (important!)
  • Small long flat blade screwdriver
  • Medium flat blade screwdriver
  • Torque wrench
  • New DOT 4 brake fluid in sealed container (1 small can will do)

The job is pretty straightforward with no surprises. If you've ever replaced a brake or clutch master or slave on an older car you'll be pleasantly surprised. If you've never worked on a car before, get someone to look over your shoulder and teach you how. This is too nice a car to screw up!

Preparation for Master Cylinder Removal

Remove the knee bolster / cover under the steering wheel. You'll find that there are 4 screws total holding it up. One is under the fuse cover (between the driver's side door and the dash) at the bottom as you face the fuse panel. Just pry off the fuse panel cover gently with your fingers; no tools required. The four screws that hold up the bolster have 8mm heads.

Next carefully pry out the two plastic covers that conceal one screw each. Remove them and work your way to the bottom right where you'll find one more screw. The knee bolster will now come clear except for 2 sets of connectors. One is the under dash footlight and the other is the socket for the diagnostic connector. Carefully pry the two clips from the light and unplug it. The diagnostic connector has a little prong which you gently depress to pull the connector out. Remove the knee bolster and set is aside in a clean safe place.

The Audi factory service manual says to remove the engine ECU to gain access to the hydraulic lines which I did, but I don't believe it's really required. I'd be interested to know if the mechanics at the dealerships really do this. If you choose to remove the ECU, disconnect the negative battery connection, but be sure you have the radio code first.

Remove the ECU

Removing the ECU is not all that complicated but it does entail the greatest risk of the whole job and is not for the timid. To remove the ECU you must find the black box on the driver's side under the hood near the firewall. You may need to remove the wiper arms and the black plastic trim under the wiper arms at the bottom of the windshield to access one of the screws. If so, carefully pry off the black plastic covers on the wiper arms to reveal the nuts underneath (12mm I think). Mark the windshield with some masking tape to indicate where the blades lie so you can line up the wiper arms again when you put this all back together. Once you have access, remove the (5) 8mm screws and carefully, with your hands, pry off the cover. Under the cover is a silver box with 2 connectors and a strap across the module. The strap comes off with a gentle pushing and prying with a screwdriver. You'll need this so try not to distend it too much when you take it out. You'll find two connectors on the module. Slide the T-handle levers out all the way and the connector will pull itself out. Set the module aside in a safe place, preferably out of the sun and dust and static. So far, so good. Now for the part I wasn't so hot to do. There are 5 connectors that need to be removed to be able to pull out the bottom of the black box that contained the module. Take your flat blade screw driver and very carefully pry the locks from each of the connectors gently pulling the connector out of its mate. Rock the connector a little front to back and it will come. Don't apply a lot of force and try not to bugger anything or this could become a bigger project that you anticipated.

Now that all the connectors are free, you can remove the (2) 10mm nuts that hold the box down and remove it from the knob at the front as well. Slide the rubber boot that surrounds the cable out of the U shaped notch in the box and the box will come free. All of this just gets you a little more clearance to work on the 2 tubes that enter the firewall here. One is from the bottom of the brake reservoir; the other is the hydraulic line that runs from the master to the slave.

Master Cylinder Removal

Whether you have removed the ECU or not, you will need to disconnect the two lines. The hose from the reservoir is a barbed fitting that won't come off easily. Be sure to clamp off this rubber hose to prevent brake fluid from pouring out all over. Behind the firewall and part of the replacement master is a plastic L shaped fitting. Mine broke as I was trying to remove the hose from the barbed fitting. You should not fret if it breaks but be prepared to keep the dribbling brake fluid from hitting paint. In retrospect, it would be easier to order some extra hose and just cut it with a knife rather that playing with the thing.

The hydraulic connection is simplicity itself. No more screw fittings. These little buggers use a wire clamp arrangement. Carefully slide the wire to driver's left and the metal line will come out of the master. Again be sure to have a rag to protect paint from brake fluid drips.


Now go under the dash. Remove the clevis pin that holds the master push-rod to the clutch pedal. Pivot it up and slide it out. Then remove the (2) 6mm Allen head socket screws that hold the master to the car. The one on the right will be easy. The one on the left will require the ball head Allen. There is a center return spring for the clutch pedal that was in the way and the ball head (aka Bondus) Allen was required.

Remove the old master and carefully work whatever remains of your L fitting from the slot in which it's retained. It's a little U shaped cut in some sheet metal that retains it. Push it up and out of this detent to remove.

Installing the New Master Cylinder

Your new master will come with a new thick rubber washer and a thin metal one. My old ones were retained under sheet metal when I removed the master. They look like they were in reasonable condition and I could see removing and reinstalling them to be a pain so I settled for keeping the old ones. I'm guessing a purist will say that this is a seal for water, so if you live where it rains a lot, look where these items live and decide if you want to fight it.

Your new (and old) master is plastic. (Go figure.) It has a piece of tubing and an L shaped plastic fitting at the end.

Snake the L fitting back to the opening from whence the old one came, line up the master on it's mounting holes and reinstall the 8mm Allen head bolts. Clean, lubricate (white lithium grease seemed right) and reinstall the clevis pin. Go back to the engine compartment, install the metal line into the master till it stops and push the wire clip back to it's original position. Replace the line from the brake reservoir on the barbed fitting. If you are only replacing the master, remove the clamp, bleed the system and button up the knee bolster, replacing the 4 screws. Not too tightly, they strip very easily.

R&R the Slave Cylinder

If you are replacing the slave get underneath the car. I have a 2 post lift, but it looks like it can be done on jack stands. On the left side of the transmission bell housing you'll find the slave and the metal hydraulic line. This line connects to the slave in the same way as to the master, with a wire retainer. Carefully slide the retainer aside. Be gentle; mine decided to fly off and it took 10 minutes of frantic looking with magnets and mirror to find it. If you lose it, fear not, a new one comes with the new slave. Again, be sure to have a rag ready to catch any brake fluid that dribbles out of the line.

Remove the one 6mm Allen head bolt and remove the slave. There is a metal bracket attached to the slave that retains the metal line. After the screw is out gently angle both the slave and the bracket to remove it from the line.

Installation of the slave was very simple, but there is some preload, probably the clutch fork, so it requires a little persuasion to line up the two mounting holes again. It'll only take your hands. Line up the hole, screw back the Allen head bolt, slip the metal line onto the metal retainer bracket, insert the hydraulic line back into the slave and move the wire retainer back into position.

Bleed the system

If the clamp on the line from the reservoir is still on, make sure all your hydraulic connections are properly connected and remove it. I use an inexpensive pressure bleeder. They can be had for about $40 on the web. You are going to bleed your brakes someday; this is a good tool and a fair price. Only use DOT 4 brake fluid. I use Castrol, but be sure to use new high quality DOT 4 fluid from a sealed can. With the pressure bleeder in place and pumped to 15-20 lbs. get back under the car and open the bleeder valve. (Use a hose and a catch bottle. Please see instructions elsewhere for bleeding brakes and clutch's.) Once all the air is out, close the bleeder, clean up and feel that new pedal feel.

My old pedal feel was horrible and the difference between old and new quite surprising. It takes much less pedal force now.

ECU replacement

If you removed the ECU, the connectors and the ECU box, now is the time to put that all back together. Put the box back in it's original position, replace the two 10mm nuts and don't over tighten. Use a torque wrench. Reinstall each of the color coded connectors into their proper location. You should hear a click when the seat, but you can also see that they are all the same height when they are properly seated. Reconnect the ECU itself and restore it to it's proper location being sure to reinstall the metal strap over it. Replace the plastic cover and replace the screws. DO NOT OVER TIGHTEN. These screws are tightened to a very low torque value. Use a torque wrench or be very gentle. You want this box to seal properly and you do not want to strip the plastic.

Replace the black trim and the windshield wiper arms using the masking tape to line them up in the right place.

Reconnect the negative battery lead. My computer did not throw any DTCs, (diagnostic trouble codes, AKA check engine light) but the Audi manual says it could. Be prepared for this. If you don't own or have access to one of the many good applications for the PC or PDA that connects to your car, you may need to take it to the dealer to clear the code or codes.

Enter your radio code. See radio owner's manual for instructions.

Reset your clock.

Please feel free to drop me a note to tell me if this procedure was helpful. I'd also especially like to hear from anyone who does the job without removing the ECU.

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