Need help with rear brake problem. 81 r100rt

[viperh]

Need help with rear brake problem. 81 r100rt (Fixed)

EDIT- Somehow debris wound up in the MC and I didn't see it. Cleaned out the M/C then pushed the fluid through the caliper and out of the M/C. Works now, will take it for a test ride in the next day or two.


I also posted this on advrider I just copied and paste. This is my first airhead.

Long story short bought 81 r100rt in order to make a hack. This was a barn find and it sat for 20+ years. I have everything but haven't attached the side car until I get the rear brakes sorted out.

I have done everything but replace the rear caliper.

First trip after overhauling everything on the bike but the brakes (Lesson #1 learned). The brakes locked up and wouldn't release. Lots of corrosion in the pressure relief hole on front Master Cylinder (M/C). Tow bike home. The rear brake seemed weak but it worked at least on that initial trip. Granted I couldn't lock the rear tire up no matter how hard I tried.

I order the whole she-bang front M/C, Rear M/C rebuild kit, all SS lines, New rotors all around, and rebuild kits for all calipers. Upon putting everything back together I stripped the rear M/C and the hardline . Now I have also replaced rear hard line and rear M/C.

I have everything put back together. Front brakes are solid. The rear not so much. I used the mityvac method to get the air bubbles out but the rear doesn't seems to be putting much pressure on the pads. I hit the rear brake lever I watch the pads press against the rotor. Then when I release the lever they back off to their starting position. I'm expecting them to stay against the rotor with no pressure on them like my new bikes.

I'm also putting the rear brake at its absolute max before the pads make contact with the rotor. In other words by the time the rear M/C articulates through the motion pretty much when I hit the point where it can't articulate anymore is right about when the brakes touch the rotor and put the pressure on the rotors.

There are two problems with this.
1. I don't think I'll be able to lock up the rear wheel at all. (granted no test ride).
2. I wouldn't be able to articulate my ankle that far with my motorcycle boots to make the brakes effective in the event of an oh **** moment.

I haven't taken it for a test ride yet but this doesn't seem right. So a couple of questions.

Can an '81 r100rt lock the rear tire up when you hit the rear brake?
Are those '81 r100rt's such a pain to bleed the lines and rotors properly that not even the mightyvac method works?
If so? What is a better method than the Mightyvac method?
Is there a newer brake system that I can just bolt on to a stock bike '81 r100rt?
Is this a symptom of a bad caliper?

I rebuilt the calipers, there was a lot of gunk in there. I verified that the orings are in the correct spots and there is no fluid leaking. Did I ef something up when I put the rotor back together? It's a really simple process. Maybe too much grease around the piston.


I figured the stopping distance of this bike wouldn't be as good as my 2010 r1200GSA. But the rear brake doesn't seem to be pulling its weight. Even on the initial test ride I tried like hell to lock up the back tire and it just wasn't locking up. With the additional weight of a hack and the extra gear I would like to have working brakes.

I'm only being extra cautious on this because on my '10 the brakes have saved my ass so many times on my commute to and from work that I'd hate play with fire.

 

[20774]

The rear disk should be able to easily lock up, especially under hard braking. With the weight shift to the front, the rear wheel would be lightly loaded and the disk can be locked up. I've read that the rear caliper must be removed from its position so that the bleed valve is at the absolute highest point. If you don't do that, you'll still have air in the system. Also, when bleeding hydraulic brakes, you should not let the hand lever or foot lever go beyond its normal travel range. Beyond that normal travel range are all manner of gunk and corrosion which will tear at the internal rubber seals. Best to put some wood or other device to limit the lever/pedal motion to prevent this from happening.

Despite the allure of a disk brake, the complexity of the rear disk does not provide significantly better stopping power than the simple drum brake.

 

[brewmeister]

Nice bike you have! I also have a 81r100rt. It sat for more than 20 years and the brakes were locked up tight as in no movement anywhere. The cylinder was honed out , o rings changed , new stainless brake lines , speed bleeders installed, new HH cindered copper pads. Master cylinder rebuild kit, honed the caliper piston. Seals where needed then go out and enjoy a nice safer ride. pm me if you would care for help with your bike as I have learned much in doing the same types of things. Good luck.

 

[Jameslf1]

rear disc brake poor

rear disc brake were always weak on my used 8/77 rs. When I got it the pads & rotor were blue glazed from heat which was due to corrsion at the outer end of master cylinder holding brake pad on. Cylinder was ok so put a kit in,no leaks & pads released but never could get a firm pedal and had lots of pedal travel before engagment. Always weak brake but had too many other things in priorioty so accepted it. Now 20 years later worked on my 84 aspencade brakes, good but untouched in 30 years. All pistons & m/c ok so wondered about the rs rear brake.
I found the rs pistons corroded & stuck but a new caliper from bmw was $500+. Found teflon coated 38mm pistons for Brembo from moto guzi for $20 a pair, also can get new caliper for around $200. Pads renewed but still soft & long pedal travel. Learned from honda & rs front problems in bleeding about air domes traps. Tilted handlebar m/cylinders & rs twin disc lite sw are hard to get air out. Noticed that all bleed fittings were at highest point except rs rear caliper which is at the bottom of caliper,piston is above = non bleedable. Loosened axle pivot nut and removed caliper bracket rod,rotated level and tightened pivot nut. Now air bleed out w low pedal travel and firm brakes for fist time. Rotor is rough but pads
quicky seated in with good brakes on rear. Can now slide wheel in some conditions. Amazing. Reduced pedal travel will reduce piston seal wear. Rs front m/c twin disc spongy regardles of bleed. Left it overnite and air rose up now w firm feel. Aspencade tilted m/c fittings must be bleed to remove air.
Found air dome problems on go carts m/c ,gm concentric clutch slave etc. but the rs rear caliper is the only bleeder valve I've seen below the piston. It's great to have a good rear brake now as the front could go out at any time, and it helps when I use the sleeper trailer.
one pecular note - the rear caliper hose & bleeder were reversed in position from diagrams & machined in recesses. I did see some early brembo rear calipers w bleeder on top outside of piston but all the rest are by the hose. Conclusion: you can have a good rear brake if you can get it bleed even w a rough rotor.

 

[dromlewicz]

1980's R100RTs will lock up rear brake when needed.

I am very late to this forum but I recently had the same issue with rear brake being weak. I did the following:

Last week I completely rebuilt the master cylinder with new seals and piston (all comes in a kit). I also rebuilt the caliper and then power bled the system with a syringe. Worked wonderfully. I went from caliper to master cylinder and then the other way several times to make REALLY sure all the air was out of the lines. There is a specific process to bleeding the system. I also have the original rubber braided hose to the rear caliper.

So I think I'll leave it as is now. I did use moly grease where the cam presses on the master cylinder piston, on all the pivot points, and the brake pedal pivot bushing. I guess it is as close to original as it ever was.

I just went on a short test ride and locked up the rear brake 3 times with normal pressure on the pedal. The first left at least 12" of rubber and I am sure the neighbors heard it.

Contact me offline and I will help walk you through it if you like.

Regards,
Kim