Porsche 944 Clutch Master And Slave Cylinder Replacement
Like many modern vehicles, the 944 series Porsche uses a hydraulic clutch actuation system. This is in contrast to a mechanical cable operated system found on the 924 series and other earlier Porsches. The reasons for using a hydraulic system as opposed to cable are smoother operation with less pedal effort, and ease of maintenance with greater reliability. I suspect that if you are reading this, you are probably questioning the notion of reliability though.
A hydraulic clutch operates much the same as the braking system on every modern car. Through the miracle of fluid dynamics, force is applied at one end of the system and a greater force is realized at the opposite end. Specifically, when you depress the clutch pedal, a rod pushes on a piston in the master cylinder which in turn transfers fluid to the slave cylinder, pushing the slave piston in outward against a lever which in turn disengages the clutch pressure plate. Because of the forces required to accomplish such a task are so great, a special high-density fluid must be used.
Like most automobiles, the 944 uses brake fluid to operate the hydraulic clutch system. In fact, the 944 brake and clutch fluid draw from the same source. Brake fluid is an obvious choice because of its unique properties and its parity with the braking system. Brake fluid is special because it is fairly dense, resists boiling at high temperatures, and is kind to the rubber seals used in these systems. The major shortcoming of brake fluid is that it tends to absorb moisture, and with it, contamination. These two combined, are what do the most to deteriorate the system. When heated to boiling temperature, the oxygen in the moisture expands creating air bubbles. The problem is that air bubbles compress much more easily than anything in the system, rendering the system useless. This is why regular fluid change intervals are recommended, and increased intervals are recommended if you live in a highly humid climate.
Like motor oil, not all brake fluids are created equal though. The range of quality parallels prices from about one dollar per pint, to about $80 per liter ( about $40/pt.). Good brake fluid usually costs about $5-$7 per pint; the more expensive stuff is mostly used by professional racing teams. The difference in quality can basically be termed as "purity". The manufacture of brake fluid is monitored by DOT and SAE (Department of Transportation and Society of Engineering respectfully) and is measured by the fluid's resistance to boiling: both wet (contaminated) and dry (not contaminated) boiling points. Both these figures should be printed on the container; if not, put it back on the self and shop elsewhere.
Just in case you were wondering, the more popular brands used by performance minded individuals are : Castrol LMA, Ford Racing, ATE (Alfred Teaves) 2000 or Super Blue, Motul 600, and Castrol SRF. Other fluids sold by manufactures of brake components are usually pretty good as well. Silicone brake fluid was once popular because it had a relatively low moisture absorption rate but its shortcoming was that it compresses more easily than standard brake fluid. This translated into undesirable braking characteristics. Now silicone brake fluid is mostly used in classic cars (which are seldom driven and rarely maintained).
Since you ARE reading this I assume you suspect or at least expect your car has some sort of clutch failure. The first symptom noticed by most owners is when the clutch peddle sinks to the floor and fails to return. This suspicion can be supported by viewing the action, (or rather lack thereof), of the throwout fork through the inspection hole. (Pic 2) Although there can be other reasons, such as a loose/broken line, or broken mechanical part, the most common failure by far is a defective clutch master or slave cylinder. It doesn't really matter which one because they are contaminated by the same fluid and should be replaced as a set; failure of the other component will likely follow soon. Further verification will be moisture or visible leaking around the system master/slave cylinder (pic 3), or brake fluid on the drivers side carpet. This should correspond with a reduced level in the brake fluid reservoir.
The real culprit for a hydraulic failure is not exclusion of fluid from the system, but rather inclusion of air. Even an air bubble smaller than a pea can disable the clutch hydraulic system. Consider this: if there is air in the system, it didn't get there by accident. It probably means it was vacuumed in past a leaking seal.
Another common, and sometimes overlooked, problem with 944 clutch system is when the mounting area around master cylinder begins to fracture. This is evidenced by excessive movement of the master cylinder when the clutch peddle is depressed. In most cases, since the area so cluttered, the actual fractures won't be visible to the untrained eye, except from inside the cabin. (pic 4)
Although it is mostly the earlier cars that are plagued with this problem, I suspect we'll see the same with the later cars as mileages increase. In any event, just be forewarned and be prepared with a contingency plan should you make this unwelcome discovery.
Our textbook example guinea pig for this project is an '84 944 afflicted will all the usual problems: a leaking master cylinder and broken firewall.
In this case, since we knew the firewall was broken, we had to remove the master cylinder first so the firewall could be repaired. Note that we are only performing one step at a time for a reason. The reason being that we want to retain as much fluid in the system as possible throughout the project. Bleeding the system will be is a real bear if the system should go dry.
The first step in this project to disconnect the battery before we forget. Not only is this a good maintenance practice, it will be a must to prevent DME damage during the welding, and electrical shock during starter removal.
Before getting to the heart of the matter you will have to remove most of the ancillary fuel injection junk from around the clutch master cylinder. This only takes a couple minutes and makes your life a lot easier. Try to keep as much of the junk intact as possible so you don't have to worry about how it goes back together. (Pic 6)
Now we can start by removing the blue feeder hose from the master cylinder. The blue feeder hose is a special long-lasting silicone hose with a cotton braided outer layer. This hose is fairly expensive (for rubber hose) and can generally only be found at German car dealerships or specialized parts retailers. Plan ahead to replace it unless is been done recently. Even though this hose was designed to far outlast conventional rubber hose, nothing lasts forever. At a minimum you will likely destroy its cotton outer layer and replacement will give the project a new and finished look.
Generally, if you plan to replace the hose, it doesn't matter which end you disconnect first. But you can save yourself a step by only disconnecting the reservoir side. A small and shallow incision with a razor blade will greatly assist in releasing the hose from its mooring. In our case we had just replaced the hose the prior week when diagnosing this problem so we removed the hose from the master cylinder end. In this case it doesn't matter if you pull out the plastic elbow from its rubber socket since your new master cylinder will include these parts. The hose was elevated and tucked over the top of the master cylinder to prevent unnecessary spillage.
Now we remove the high pressure (hard) line fitting. The reason we do this before removing the master cylinder retaining nuts is because brake line fittings can be pretty stubborn if not maintained regularly (which is most the time). Essential in this process is the use of a 12mm "line wrench". A line wrench is characterized by its heavily shanked 6-point box-end with an opening designed to (just) fit over the tubing (pic 5). Use of anything other than this specially designed wrench will most likely result in a damaged fitting, unnecessarily complicating this task.
We can now remove the master cylinder retaining nuts. This would probably be pretty straight forward if not for the fact the nuts are in a pretty cramped area. The best tool we found for this is a 13mm swivel head socket and a long extension. (pic 7)
Even though your new master cylinder comes with a new push-rod, these parts seldom wear out or need replacement (or adjustment). For this reason we chose to save some time and leave the old push-rod attached to the clutch pedal while we replaced the master cylinder.
Before sending the car to the welder we removed the driver's seat, carpet, and loosened the steering wheel so the welder could have room to work under dashboard. (pic 8). We also used a heat gun and screwdriver to loosen the sealant from around the area to be welded. (pic 12)
Once the car was returned from the welder we were ready to install the new master cylinder. But first we applied some paint the fresh welds to protect them from rust (as least for a while). Por-15 or Eastwood Rust Encapsulator is good for this task.
Before we could install the master cylinder we had to "prime" it. This involves filling the master cylinder to capacity while expelling as much air as possible. This process is most effectively done by submerging the master cylinder in fresh brake fluid and pumping the piston with the pushrod till air bubbles cease to exist. But this method is messy and wastes allot of fluid that can't otherwise be reused. I prefer to use a syringe.
Normally master and slave cylinders are shipped with caps in the orifices to protect the innards from contamination prior to assembly. In order to prime the master cylinder the caps must be removed, but keep them handy as you will need them after the priming is done.
The priming is accomplished by holding the master cylinder with the orifices on a level plain and the piston fully depressed with the push rod. Have an assistant slowly fill one of the orifices while drawing out the piston. Once the piston is fully drawn out, push it in again slowly till a steady stream of fluid comes out the fitting end. Again, add more brake fluid as you slowly draw the piston out. Repeat till the air is expelled and replace the caps to reduce spillage while you work. (pic 9)
Now, because the caps were never designed to be leak proof, you will need to work rather smartly (before the fluid leaks out). Place the master cylinder into position, but don't nut it down. Assuming the blue feed is already attached to the reservoir, and the brake fluid reservoir is topped off, remove the inlet cap (the one on top) from the master
cylinder. Allow the fluid to flow down the (blue) feeder hose as you lower and install it to the master cylinder (top off brake fluid again). Once secured you can remove the fitting cap from the (pressure) end of the master cylinder and screw in the line fitting. These line fittings tend to be somewhat temperamental and pretty much require a straight shot in to get the threads started; then you should be able to snug up the fitting by hand. Use you line wrench again to secure the fitting. This is where allot of make the mistake of over tightening: just because the fitting was a bear to get off doesn't mean it needs to be installed with the same veracity; 10-15 ft/lb should be enough. (Top off brake fluid again).
Now, before you tighten down the master cylinder you will need an assistant under the dashboard to guide the push rod into the master cylinder boot as you do the final positioning. (pic 11). I personally always keep a supply of metric stainless steel hardware on hand; so we used stainless for the master cylinder retaining nuts to overt difficulties with removal in the future. (pic 10)
All the work is from the top side of the car is now complete so the car can be jacked up to replace the slave cylinder from the bottom. Don't forget to safely place the jack stands under sturdy suspension components. In our case we only jacked up the left side of the car.
After removing the starter (make sure the battery is disconnected) and tucking the wires out of the way, I use the 12 mm line wrench once again to remove the feed line from the top of slave cylinder (pic 13). At this point I only break the line loose to save myself the difficulty later. The line was also rusted to the compression nut and I didn't want to risk breaking the line buy turning the nut against it. Watch for this on all similar lines. Next I use a 13mm swivel socket to remove the two mounting bolts. (pic 14) This can be done with a standard 13mm wrench, but it takes a while since the bolts are pretty long.
Next I compress the push rod and wrap with high strength tape. The reason I did this was to maintain pressure in the system and overt the intrusion of air. (pic 15) Now its time to prepare the slave cylinder for installation. This involves filling the slave cylinder with fluid the same way we did with the master cylinder. Once the new slave cylinder is ready we can quickly remove the old one and replace it with the new one. At this point the compression nut only needs to be finger tight.
In our case, since the compression nut was frozen, we ended up unscrewing the slave cylinder from the nut, as opposed to unscrewing the nut. We then used a silicone plug in the line temporarily while we freed the nut with a little Liquid Wrench.
Once the feed line is connected we can reinstall the slave cylinder. We now can install the two M6 (13mm) bolts, but only turn them in by a few threads. Now crack open the bleeder screw and close it just tight to the touch. This will allow sort of a self bleeding action as we tighten the slave cylinder down. Once the Bolts are tightened to about 18 ft lbs you can tighten the compression nut and the bleeder screw.
Now comes the hard part: bleeding the air out of the system. If all the preceding was done correctly you will have little to no air remaining in the system (such was our case). In any case you will need to bleed it to be sure all the air is expelled.
Bleeding the clutch is similar to bleeding the brakes. You only need to pump the clutch peddle 2-4 times, hold to the floor, and have your helper crack open the bleeder screw momentarily. Once the bleeder is closed you can repeat the process till all the air is expelled from the system. Be sure to keep the brake fluid reservoir full at all times. If you depress the clutch peddle and it fails to return that means you have too much air in the system. This is a problem. People have tried just about method under the sun to bleed these clutch systems but the only thing that really works with consistent success is pressure bleeding. Barring that you'll need to find a way to at least get enough fluid into the system that it can be pumped up with the clutch peddle. Then you can bleed it normally.
After the system is properly bled you can reinstall the starter. The only thing reaming to do is to adjust the clutch peddle free play. This is done by climbing under the dashboard again and loosening the jamb nut on the push rod. Turn the pushrod clockwise (to tighten) till it barely contacts the master cylinder, then counter clockwise one or two turns (doesn't have to be exact). Check the operation of the clutch so that the clutch begins to engage about an inch or better off the floor. Once your happy with the adjustment you can tighten the jamb nut and take the car for a well deserved spin around the county.
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