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1972 Porsche 914Repair and Restoration - Part 16 - Rear Brakes5/30/2009Ok, first let me tell you what my plan is with regards to the rear brakes. First you may want to read up on what I am doing with the front brakes. The short version is that I am using A-calipers from a 911. These use 48mm pistons and should be nice brakes. You should always make sure you front and rear calipers are compatible with each other. It seems odd that you need to do this as they are on opposite ends of the car, but what you need to proper front/rear balance. So for example if you put extra large calipers on the rear, you would end up with a balance that is toward the rear and would result in a tendency to lock up the rear wheels prior to the front. That is a dangerous combination. You can try to find the proper front rear balance on your own or you can go with a proven combination. I am using the exact same calipers that Porsche put on the rear of 911s that had the A-calipers on the front. This is a proven combination. So I am using what is generally known as "rear M-calipers". Now is the time to mention that the rear brake setup on a 914 is not like that of a 911. The main difference is the parking brake. The 911 uses a drum style system for the parking brake with the brake rotor working both with the drum parking brake and the rotor for the caliper. The 914 uses a special caliper that combines hydraulics for normal braking and a mechanical system for the parking brake. But both use the rotor and don't use a drum style for the parking brake. So if you move to the 911 caliper, you loose the 914 parking brake setup. There are a number of solutions to this, but the most common is to modify the 914 rear swing arm to accept the 911 parking brake hardware. This is generally an easy modification. The hardest part is the cabling system. That is because how the 914 parking brake cable is mounted, it's length and how it terminates is different than the 911. This results in many solutions that range from a replication of the 911 cable, to systems that use a bell-crank to some newer use of 944 parts that have a similar parking brake setup. I plan to use the 911 parking brake with some of the 944 parts. I mentioned this on the front brakes page, but it's worth mentioning here as well. I plan to do all of my own work, but if I wasn't, I would have shipped my calipers off to Eric Shea at PMB Performance... Even if Eric is not going to be rebuilding my calipers, he has been gracious enough to answer some questions for me. The first thing I noticed is that he pistons in my M-calipers are pretty rusted in the area around the dust seals. I emailed the photo below to Eric and he feels that they are reusable once cleaned up, but I may end up deciding to buy new pistons from him before I finish the rebuild process.
5/31/2009Ok, I pulled apart one of the calipers yesterday, but didn't take any photos other than of the rust on the pistons. here is a typical rear M-Caliper. Note that this uses the later style pistons (they are deeply cupped). The earlier style does not have the deep cup and had a built in "knock back" mechanism. The later are considered to be slightly better as the piston does not heat up as quickly as the earlier style piston.
Read all about how to disassemble these calipers on the front brakes page. The process is pretty much identical. I will mention that these use M7 hardware instead of the M9 hardware of the A-Calipers. M7 is just as impossible to find as the M9. The only upside of the M7 bolt is the it also uses the 6mm Ribe head, but with the smaller M7 nut, it's less likely that you will have problems removing the nut as you might have with the M9 hardware. You still need to heat up the nuts, let them cool, etc. as I describe on the front brakes page. |
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