I realize the brakes on the airhead leave a lot to be desired but anyone figured out how to make the rear disc on a 77 R100S work? Caliper change? Master cylinder change? Mine won't lock the rear wheel if I stood on it with both feet. Not really willing to go to drum but looking for a fix.
It is a REAL job to get all the air out of that system. I suspect that is your problem. That and/or a shot rubber line.
It could have been made from 9/77-12/77. The only way to get the rear disc to work is a smaller bore master cylinder like a 12mm. Modify the mounting to use an earlier K100 with ABS, R1100 or R1150 rear brake master cylinder.
New brake hose is a must on these old Airheads. The originals have all swelled up and choked the hole down the centre by now. And that rear brake is a bitch to bleed for some reason. Taking caliper off and hanging it so rubber hose is vertical helps, as does a vacuum bleeder. Can't remember exactly but I think there were two sizes of master cyl bore, the later ones being smaller and more effective. You can get the old ones sleeved to fit the later piston/seal.
Good advise you guys! Yea, the master cylinder bore is too big on those but still . . . the smaller MC's don't work well with junk lines full of air either! I really was going to say something about that but I though about first things first.
I never liked the rear disc on my 78. Seemed like way too much crap not doing 1/2 what the drum could do. Plus it made getting the wheel off more difficult. But it sure looked good.
Had the same priblem with my 81 RT a few years back.I ended up replacing the master cylinder,new steel line,and a steel braided line to replace the rubber line.I then added a little wxtra line off the fluid resivoir so I could rise up above the master cylinder and then pumped the hell out of the pedal till I had a real brake..It was apain in the asss but it worked.Your right it does look good
what have worked on my RS : I put old break pads, a crew driver between break pads. I pupm on break pedal Twist the larg flat screw driver, brake fluid and air goes back in brake fluid tank. I do again and again that
The bike is a 78 from the engine number I have replaced the rubber hose with a top grade one from a place in Ohio. New pads and bled the hell out of the system. Roughed up the rotors a bit with some emory cloth and cleaned out the cooling holes. So it looks like a rebuild of the master cylinder and getting the rotor turned
Two mates of mine replaced their rear steel brake line and rubber hose with a custom one long stainless steel brake hose. Both are very happy with the solution. This comment was as significant for the rear brake as it was for the front.
You can waste as much time and money and thought on this as you want. Unless you go to a smaller bore master cylinder nothing will really change. These things were crap when they were new.
..crappy as a dookati's I might add, even with a steel brakeline. Wished I had the drum brake off my '78.
