Rebuilding the Rear Master Cylinder

I have received the rebuild kit for the rear master cylinder (HD P/N 42810-04B) a few days ago. The kit contained the following items:

• 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.

Rear Master Cylinder Failed!

Well, it happened. The rear master cylinder on  my Sportster failed at 41,600 miles. It really wasn't all right since the rear pad change at 30,000 miles. The rear brake has been weak and required a bit pf pumping once a day to get the pedal to not sink too far down. Once pumped up it would stay up for a relatively long time (days). The rear brake was not totally mushy, but it was not all that strong. I could still lock up the wheel, but with more effort than I thought was necessary. Anyway, since the brake was sort of serviceable, I left it alone until an opportune moment to deal with it. Well, that moment arrived a few days ago when I removed the rear wheel for a tire change. I get my tires online at Motorcycle Superstore which has the best prices on tires for my bike, BTW. I then remove the wheel assembly and take it to a good shop to mount the new tire. So, while the wheel was off the bike I decided to bleed the rear brake and get rid of that funkiness.I hooked up my MityVac and proceeded to vacuum bleed the brake via the bleeding screw on the caliper, which I removed from the swing arm and hung up such that the bleeder screw was the highest point in the system. This causes air bubbles to collect at the bleeder and not in that arch the brake line makes when the caliper is on the swing arm. If you own a rubber mounted Sportster you know what I mean. Anyway, after bleeding I had NO REAR BRAKE AT ALL! WTF?! Well, I read up on various forums, but mostly here that this is a rather common issue. After changing the pads and pushing the rear caliper cylinder back to accommodate the new pads many, many, MANY people observe the failure I had. Even the pictures look the same as what I found when I disassembled my rear master cylinder. I have ordered a rebuild kit and will rebuild this thing and see how things go.