Master Cylinder Replacement

This procedure was written after this job was done on a '74 Karmann Ghia. It should apply to all Karmann Ghias with front disc brakes and should apply to standard Beetles from around 1967 to 1977. This procedure is for the dual-circuit master cylinder fitted to late air-cooled Volkswagens.

Be aware that the master cylinder itself is different for disc brake Karmann Ghias than it is for Beetles which have four-wheel drum brakes. Make sure that you get the correct part for your car.

TOOLS NEEDED: Lots of brake fluid (DOT 3 or DOT 4), bulb syringe, 7/16" wrench, 13mm wrench, adjustable wrench, jack, jack stands, 19mm socket and ratchet, hubcap puller, wheel chock, glass jar, piece of flexible plastic tubing (preferably transparent) that will fit tightly over the brake bleeder valves, and a light. There may be things I'm forgetting here.

This is a fairly straightforward, though messy, job. First go out and buy your materials. You need the bulb syringe to siphon brake fluid from the reservoir. You can find them in the kitchen utensils section of Wal-Mart for $1. You also need a lot of brake fluid, at least 2 32oz bottles. Also get the master cylinder. I found mine at Trak Auto. It's a rebuilt unit with a lifetime warranty. It cost about $20. I recommend you buy new German if you can get it. It will probably cost more than $20 though.

The first step is to block your wheels, remove the hubcap from the left front wheel and then loosen the wheel bolts a bit. Jack up the car and place a jackstand under the front axle beam. Remove the wheel and lower the car so that the axle beam rests on the jackstand. At this time you may wish to turn the steering wheel fully to the right to give you better access to the master cylinder.

Look into the left front wheelwell. Deep in there you see the master cylinder attached to the front bulkhead near the center tunnel. You see three metal brake lines coming from it, one to the right front wheel (this one comes out of the top of the master cylinder), one to the left front wheel (the frontmost one on the side) and one that goes back through the bulkhead (the rearmost one on the side). The one that goes through the bulkhead is for the rear brakes. It is a single line here which splits into two lines later on.

You also see two pressure switches below the two side brake line connections. These pressure switches control the brake lights at the rear of the car and also the brake warning light on the dashboard. You also see two hoses coming from above that connect to the top of the master cylinder. These hoses are the fluid supply hoses leading from the reservoir in the luggage compartment.

Open the front decklid and remove the cap from the brake fluid reservoir. Find an empty container of some sort (NOT the glass jar you will be using to bleed the brakes) and your bulb syringe. Your job here is to siphon out as much of the fluid in the reservoir as possible, to reduce the amount of spillage you will have. Once you have as much fluid siphoned out as possible, head back down to the master cylinder area.

You want to remove the supply lines from the master cylinder. You may wish to spread some rags down on that shelf below the master cylinder because you will spill as much fluid as is in the lines between the reservoir and the cylinder. You will notice that there are plastic pieces coming up from the master cylinder that the hoses are attached to. I found that it was easier to simply pull up hard on the hose and plastic together and remove the plastic pieces from the master cylinder than it was to remove the hose from the plastic piece. My rebuilt master cylinder came with new plastic pieces already installed, so it was no problem. Just pull them out of the top of the master cylinder and watch the fluid run out. Don't worry if you break them, your new master cylinder has them. Hopefully.

Next go ahead and disconnect the electrical plugs from the brake pressure switches. Grasp the boots and pull back. The electrical plugs are usually pretty firmly wedged inside the boots and should come off with the boots. Lay them aside as best you can.

Now it's time to disconnect the metal brake lines. Get your 7/16" wrench (this is what size mine were, nothing metric seemed to fit the nuts well) and loosen the union nuts. They unscrew counterclockwise.

If you get one that absolutely won't budge, you may be in for some fun. I had this problem when I did this job. The nut for the line that goes to the left front wheel would not break loose. Eventually I just rounded off the nut. If this happens you will have to replace that section of brake line. Luckily for me this was the left front section, which is the shortest and easiest to replace of all of them. If it had been the nut for the rear section, I would have been hating life. What I did was loosen the nut at the other end of the line, where the metal line joins the rubber hose. I just removed the master cylinder with that piece of line still attached, then went to the local foreign auto parts store and bought a section of brake line. The one I bought was the shortest one they had, but it was still quite a bit too long. But I was able to bend it so that it fit nicely. You may have to do something similar if you wind up with a rounded nut or otherwise damaged metal line.

Once you have all the nuts loose, pull the lines out of the master cylinder and just let them sit. Ideally you should cover the open lines so that dirt doesn't get inside. You don't have to worry about fluid draining out if you leave them open, however.

Now you need to move inside the car to remove the two bolts that hold the master cylinder to the frame bulkhead. You will find the bolts forward of the brake and gas pedals, one on either side of the master cylinder pushrod connected to the brake pedal. They are 13mm bolts. Get a light, your head, and a wrench in that space comfortably and you are a better man than me. It helps to move the seat back as far as it will go. Remove the two bolts and pull them out carefully. Each bolt has a washer and a big spacer on it. Be careful not to lose the spacers. Make sure you pull the bolts out slowly with the spacers hanging on.

Now the master cylinder is ready to fall out. Go back to the wheelwell area and pull it out. Mine was a bit sticky but after some wiggling it relented and came out. Of course it was also the original master cylinder which had been there for 25 years. You will find that the boot comes out attached to the master cylinder (this is the boot that goes around the pushrod). Pull the master cylinder out and set it down on some sort of rag so you're not dripping brake fluid all over your floor.

Now if you left those plastic pieces inside the supply hoses when you pulled the master cylinder out, now you have to remove them from the hoses. What worked for me was to get a pair of pliers on the plastic and apply a twisting motion as you pull. They'll come out eventually.

Now your job is to remove the two pressure switches from the master cylinder. Get out your adjustable wrench and go to it. These also unscrew counterclockwise. Once you get them out, clean them up and set them aside. You will be re-using them, unless you have bought new ones to install. If you didn't buy new ones but you think you'd like new ones, go for it. They are fairly inexpensive.

Remove the rubber boot from the back of the old master cylinder and clean it up. Then set it aside. If your new master cylinder did not come with a boot (mine did not) then you will be re-using it, unless you bought a new one to replace it with.

OK, this is a good time to grab lunch and a beer. Wash your hands first, that brake fluid is nasty stuff.

OK, now that you are properly refreshed, it's time to start the installation. As a first step, install the clean brake pressure switches on the new master cylinder. Remove any rubber or plastic dust plugs first, please. Tighten the switches down good and snug with the adjustable wrench. Now install the boot to the new master cylinder. It simply slips over the ridge on the back of the cylinder.

Now you need to mount the master cylinder to the frame bulkhead. Insert it into the opening in the bulkhead. Make sure that the pushrod attached to the brake pedal goes into the opening in the boot so that it's pushing on the master cylinder piston. You may need to place something under the front of the master cylinder so that it will remain level while you go into the car to insert the bolts. I used a screwdriver handle. If you do this, make sure that you stop tightening the bolts after you get them started in their threads and go remove the screwdriver. I forgot to do this and found my master cylinder tightened down with the screwdriver trapped firmly between the master cylinder and the shelf below it.

Go back into the car and insert the bolts and tighten them down. Go ahead and tighten them all the way down, you won't need to loosen them later.

Now that you've done that, you need to attach the fluid supply lines to the master cylinder. They just push on to the plastic pieces. Now you may have to bench bleed the master cylinder. I say may because I did it because the instructions I got with the master cylinder said to do it, but I don't think it was really necessary. If you wish to do it, put a little fluid in the reservoir and push the brake pedal down about an inch, then release it. Do this a couple times. The idea is to get air out of the cylinder and fluid in so that when you try to bleed the brakes later, the master cylinder won't be airlocked. I think there's a more effective way to go about this, which I will explain later.

Next step is to reattach the metal brake lines. Simply insert the lines into the proper holes and tighten the union nuts. Union nuts are tricky, you have to get the nut and the hole in the master cylinder perfectly aligned before the nuts will start on their threads. Be persistant and if you're having trouble, try pushing the brake line around to give you a better position for the union nuts. Again, snug the nuts down good and tight, but don't overdo it.

Next you can plug in the electrical plugs for the brake pressure switches. This is pretty straightforward.

Now you're ready to start bleeding the brakes. Take your glass jar and fill it about a quarter full with fresh clean brake fluid. Then fill the fluid reservoir to the top with clean brake fluid. Start with the left front wheel, since it's the most convenient to your current position. If you have disc brakes, you will note that there are two bleeder valves on the caliper. The lower one is a drain valve and the upper one is the bleeder valve (since air rises). Go ahead and open up the drain valve and let the fluid ooze out into your glass jar. It will likely be dirty and dark. You don't need to open up the bleeder valve much. If you open it too much, fluid can leak out around the threads and air can leak in. Open it just a little, usually a quarter to a half turn works fine. Fit your plastic tubing to the bleeder valve, making a tight seal. Put the other end into the fluid so that it stays submerged. If you have a friend, call him or her over. If not, you can do this yourself, it's just less convenient.

You want to pump the brake pedal so that fluid flows from the bleeder valve into the jar. The idea is to keep pumping until the fluid coming out is very clean and there are no bubbles in it. If you are pumping the pedal and pumping it and you feel no resistance and nothing is coming out of the bleeder valve, the master cylinder is probably airlocked. To fix this, loosen the union nut for the appropriate brake line a bit. Have someone push on the pedal as you watch what oozes from the connection. When bubbles stop coming out and only fresh fluid is oozing, tighten the union nut. When you are doing this, you want to have the other person press on the pedal and hold it as long as the connection is open. Close the connection before you have them let up on the pedal so that no air is sucked back in. You can repeat this on the other union nut connections at the master cylinder as you feel necessary. Note that if the master cylinder is airlocked, you will get a lot of air out and then fluid. This will make the fluid level in the reservoir drop. Make sure you top it up before you continue bleeding the brakes.

Back to bleeding the brakes. You are still on the bottom bleeder valve on the left front caliper. Once the fluid coming from there is clean and bubble-free, close that bleeder and move the hose to the upper valve. Be sure to keep the other end of the hose submerged in the fluid at all times while bleeding, so air cannot re-enter the system. Also be sure to check the amount of fluid in the reservoir regularly and top it up as needed. If air enters the system because the fluid ran dry in the reservoir, you will have to start all over again. It helps to keep a larger container handy to pour the contents of the glass jar in when it gets full.

Basically to bleed the brakes you repeat the above bleeding procedure at each wheel until you get clean bubble-free fluid at each bleeder valve. In the rear, there is only one bleeder valve per wheel. Also Beetles with front drum brakes will only have one bleeder valve per front wheel.

Now that the bleeding is done, make sure all the bleeder valves are closed and then check the brake pedal feel. It should feel firm, not spongy. If it feels spongy or the pedal slowly sinks to the floor, you still have air in the lines and you need to go back and bleed the lines some more. If it feels fine to you, then go up front and check the master cylinder for leaks. If there are none, go up and top off the fluid reservoir. Then you can put the left front wheel back on and lower the car.

You're not done yet, though. You need to verify that the clearance between the master cylinder piston and the brake pedal pushrod is correct. This clearance should be 1mm. You can't measure it directly but Bentley states that the correct clearance of 1mm corresponds with a 5-7mm movement of the brake pedal before the pushrod contacts the piston. So take your brake pedal and slowly move it like you're pushing on the brakes. Feel/hear when the pushrod contacts the piston. If the distance the pedal moves before contact is made is more than 5-7mm (it was on mine) then you need to adjust the brake pedal freeplay. There is a pedal stop bolted to the car floor immediately forward of the brake and clutch pedals. There is a range of adjustment to this stop, it moves forward and backward. If you need to adjust it, get a 13mm socket on a ratchet and extension and loosen the bolt that holds the stop in place. If you need more freeplay, just pull the brake pedal back toward the rear of the car (this should move the stop forward). When the play is correct, tighten the bolt for the stop back up. If you need less freeplay (my case), loosen the stop bolt, then take the slack up in the brake AND clutch pedals. Push them both forward at the same time (the stop stops both pedals, so if you just move one, you won't be able to move the stop) and then push the stop rearward until it's in the right place. When freeplay is correct, be sure to keep holding the pedals pushed forward and then tighten the bolt for the stop. Don't let the pedals go until the bolt is tight or they can move the stop back to the original position. Now verify that you have 5-7mm freeplay on the brake pedal and 1/2" freeplay on the clutch pedal. If your clutch pedal freeplay is incorrect, now is the time to adjust it at the clutch end as per Bentley.

You can actually adjust the pushrod length by means of a locknut on the pushrod, but Bentley specifically says that you don't need to do it and you should not do it. Take it from me, DON'T adjust pedal freeplay by altering the length of the pushrod. I learned the hard way on the '75 Beetle I used to have. I messed with that adjustment before. When the brakes got hot the piston blocked the compensating ports internal to the master cylinder (due to my adjustment) and the brakes locked up. That's bad -- plus it ruined the master cylinder in the car. Only adjust pedal freeplay with the pedal stop bracket.

OK, now you've got the master cylinder installed, the brakes bled, the fluid reservoir topped off, and the pedal freeplay adjusted. Clean up your mess in the garage, dispose of all the brake fluid you passed through the system in a safe manner. Under no circumstances should you reuse any brake fluid. Make sure the fluid cap is on the reservoir, and go for a test drive. Before you get up to speed, verify that the brakes are working. Then take it out on the road to test the function of the brakes. Once satisfied, come on home and have dinner and another beer and congratulate yourself on a job well done. If your master cylinder was rebuilt then you probably need to take your old master cylinder to the shop where you bought your rebuilt so you can get your core charge refunded.

That wasn't so bad, was it?

Sean Bartnik
March 4, 1999

Back to the tech page.