- new primary seal (cup)
- new piston
- new piston (secondary) seal
- new piston return spring
- new C-clip for the pushrod retaining washer
- new pushrod rubber boot
- new spring pin
- a pouch of grease
- a pouch of brake assembly oil
- a set of instructions on how to use the kit
Upon examination of old vs. new parts I immediately saw a defect in the old piston return spring that would explain the primary cup failure. If you look at the pictures, the new spring is on the left and the old is on the right. The new spring has a very flat surface at the top end. This is the end that is in direct contact with the primary cup. The little nub on the cup fits into the hole in the spring and the piston pushes the whole thing from the other side of the cup inside the bore. Look at the old spring's mating surface. It has high points that will create uneven pressure on the rubber cup and eventually wear cracks in the rubber, which is exactly the mode of failure here.
I am not sure if something happened to the spring to create this uneven surface or it was like that from the factory, but I can definitely see how this will cause problems with longevity of these cups. Every rear MC failure (talking rubber-mount here) I have seen out int he Sportster community had this cup tearing in a very similar way. It may be that excessive pumping during the initial bleed of the system compresses the spring to the solid state (coils binding) and then the pressure from the piston collapses the thin sheet metal of the flat mating surface around the spring wire underneath. I am just guessing here. The only way to know is to take my MC apart in a few thousand miles and see what the insides look like. I may do this in the future, but for now I am just trying to get my bike back on the road.
I found that using the rebuild kit was simple and the instructions are very good. I cleaned the cylinder with rubbing alcohol and let it dry. Then I used the provided oil to lube all the rubber parts and assemble everything. I used the new C-clip even though the old one was perfectly fine. Everything went together very easily and moved smoothly.
I bled the MC by mounting it back on the bike and reconnecting the fluid supply hose from the reservoir, but not the brake line. I waited a little until the fluid started to drip out the brake line port of the master cylinder. I then pumped a few times (with the brake line still not connected) and using my finger as a bleeding screw. I would open the port before pushing the pedal, push the pedal all the way, then close the brake line port with my finger and release the pedal. I did this several times to make sure there is fluid in the cylinder bore. I then reconnected the brake line and used a vacuum pump to bleed. I did this with caliper suspended in a highest position (see pictures in my previous blog entry). After some fluid has passed through the system using vacuum I switched to the good old method of coordinated pedal pushing and bleeder screw open/close moves. This got most of the air out. However I still had spongy pedal. I complained about it to an indie mechanic who was giving me an annual safety inspection (mandatory in my state) and he suggested I push the caliper piston all the way back in and then use the pedal to get it back out. I tried it and the pedal feel improved. So far so good. I will keep my fingers crossed and perhaps try bleeding some more after I put some miles on this whole thing. The brake is definitely better than before, though I am still not impressed with its power, but that may simply be design.
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