If your brakes feel spongy, and you recently bled them, and replaced the flexible brake lines, then it's quite possible that you're master cylinder has worn out. Although it would seem logical to rebuild your master cylinder, newer and larger ones are relatively cheap (around $100).
The 914 and very early 911s originally came with a 17mm master cylinder. The 914-6 and later 911s came with a 19mm master cylinder that allowed the driver to apply more direct pressure to the brake system. The 19mm unit is externally almost identical to the 17mm unit, and is a direct bolt-in replacement. Pedal travel is reduced a little bit, but I never really noticed any discernable change. The more direct result is a bit more stomping power on the pedal.
Although this article can be utilized for both the 914 and the 911, we'll talk mainly about the 914 replacement, since it's my 914 in all the pictures. Generally, the replacement procedure is the same for the 911.
The first step in replacing the master cylinder is to bleed the brake system dry. You can place a pressure or vacuum bleeder on the system and simply open one of the bleed nipples on one of the rear calipers. Let the system run dry. There should be no brake fluid in the reservoir inside the front trunk. Have a roll of paper towels on hand, as this can be a messy job. Also, be very careful not to get any brake fluid on the car's finish, as this will eat the finish off of the paint. If you do happen to spill some, then don't wipe it - blot it instead, because you will just increase the damaged area.
After the brake system is bled, remove the belly pan from underneath the car. This pan covers and protects the master cylinder, gas lines, and steering rack from road debris. The master cylinder can be seen in Figure 1 after removing the belly pan. After the cover is removed, remove the electrical connection to the pressure switch on the side of the master cylinder. This can be seen covered by its rubber boot in Figure 2.
Now remove the brake lines that are attached to the master cylinder. Becareful when loosening the nuts, as they are usually fastened very tight and won't relent very easily. You may want to invest in a crescent wrench that wraps around the entire brake fitting to avoid stripping. If you strip the lines, you can still obtain replacement ones. After the brake lines are detached, then remove the reservoir lines from the top of the master cylinder. These simply pull out from the top. If it is really difficult, you may want to do this after you detach the master cylinder from the car.
After all the brake lines are removed, then loosen the two nuts that hold the master cylinder to the car. The studs that stick out of the chassis are directly attached to the pedal cluster inside the cockpit. After the master cylinder is removed, the car should look like Figure 3.
Installation is a bit more difficult than removal. The tough part is getting the reservoir fittings to properly seat into the new master cylinder. The new master cylinder should have new rubber boots that surround and secure the reservoir lines. In Figure 4, you can see how the lines are supposed to wrap around the rubber grommets. After doing this twice, I have not yet figured out a completely painless method of inserting the lines into the master cylinder. If you remove the rubber grommets from the master cylinder, place them on the ends of the reservoir lines, and then insert them into the master cylinder, you can usually tell if they have slipped, or if they have seated properly. The toughest part seems to be getting enough leverage when your crouched underneath the car.
After the reservoir lines are inserted into the master cylinder, then attach it to the chassis with the two nuts. Then reattach the brake line fittings. They do not need to be as tight as they were when they came off. Reattach the electrical switch. The reassembled master cylinder is shown in Figure 5.
Now rebleed your brakes. Everyone seems to have a different method of bleeding. I personally have had the best luck with the old fashioned method of stomping on the pedal with someone turning the bleed nipples. Some people think that this is the only way to properly bleed the proportioning valve in the rear of the 914. There's a good article about bleeding 914 brakes in the January '98 (#74) issue of Excellence.
I wouldn't recommend replacing the belly pan until you are completely happy with the brakes. Also, if there is not enough brake pressure, the electrical switch will trigger and need to be reset by pressing the little button on it's side. This tip is explained in detail in our Q&A section.
Well, that's about it. If you have any questions or comments about this article, please drop us a line. Bill Kohnke adds the following:
Hey Guys the 19 MM cyl looks great fits just right
A suggestion possibly you should suggest new copper gaskets for the one line to the front brakes take it apart there and it's easier but I do like to always use new in any brake job. To remove the master cylinder I removed the "Banjo" fitting rather than remove the brake line from that. This fitting has copper washers that are the gaskets in a brake system. I always like to use new washers when working on brakes.
The process went easy especially in having a New Mexican car All fittings come apart with no rust/problem, You ought to see what we go through with a car that is only 4 years old and winter driven (9 1/2 months of our year.
Another tip: Bolt the master Cyl up to the Floor then get ready to put the rubber gasketed filler lines in the top of the Cyl.
If you take a very small but long screwdriver,get the cheap one, flatten the end in the grinder then bend the end over in a U shape. (that's why the cheap screwdriver! anything good at all and It'll snap off) Hold one side canted in the hole with one hand and use the blunted end to work the filler tube and gasket down into the intended borefrom the other side.
Only takes 1 Min for each gasket broke my heart.
Thanks Bill Kohnke
Scott Guthrie adds:
Thanks to your article, my installation of a 19mm master cylinder went really well. My one contribution is that, if you're working in a quiet garage (okay, the radio wasn't _really_ loud) you can hear/feel a *snick* when pressing the feed lines into the rubber grommets if they're still in the master cylinder. A couple push-pull-twists to make sure they were seated and I was done!
Shawn O'Brien adds:
I have skinny arms and still could not get the reservoir lines in to the master cylinder either way that was suggested. I decided to unfasten thebrake fluid reservoir from its clamp and slide it down in between the fuel tank and wheel well. This gave me enough slack in the hoses to push them into the master cylinder grommets and then fasten the m/c to the floor. Refasten the reservoir and get ready to bleed.
Master Cylinder Installed
Master Cylinder Installed with Brake Lines Shown
Master Cylinder Removed
Reservoir Line Fittings
Master Cylinder Reinstalled