Got an '06 Sportster 1200 Custom I've got 90% put back together after tearing it apart, getting a new, different style tank, all bodywork off to the painters while I installed a dual Hawg Halters disc braked Frankenstein trike conversion under it. Guy told me I'd probably need a larger rear master cylinder - to that point I'm thinking their right. I've bled and bled - bled by pump pump-open/close brake valve... I've used a toomey syringe to suck fluid throughout the whole system... I've gone to a trike conversion builder at a Harley dealership about 90 miles away - tried his trick... still no rear brake pedal resistance what so ever. This bike don't have the brake master cylinder all in one off on the right side like the wife's '01; it's got plastic crap underneath in front of the rear tire area or else I could easily get a 3/4" cylinder like the wife's off line. Anyone know of a 5/8 or better yet 11/16 bore remote reservoir master cylinder off another Harley model? All parts guys I've met only know part numbers. These rear brakes is all I've left to do. Just started it for the first time all summer just to hear it run with the new vanes shorts. (freaking paint shop took 2.5 months! I was like WTF?!)

 

Sorry to hear about your problem. Unless you are towing a trailer those dual discs are more trouble than they're worth on a Sportster.

I have the single disc on mine and it stops better than it did as a two wheeler. The other day I had to make a quick stop because of a car turning in front of me and I was amazed at how quick it stopped. As a two wheeler I'm sure I would have locked up the rear wheel but as a Trike it just stopped without locking up and it was QUICK!

I've read about others with your problem which is another reason I decided to stay with the stock Frankenstein single disc. The fix for some was a new master cylinder because the stock Sportster master cylinder was never intended for two separate brakes.