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79' Rs rear disc brake
Posted: Sun Apr 22, 2018 5:27 pm
by bluesman
I have a 79' R100rs with rear disc brake. I have good pedal but it has little to no stopping power. I have heard that they were not very good to begin with but it seems they should have a little more stopping power than what I have. The pads and rotors look OK, Is there anything I can do to improve the performance?
Thanks
Bluesman
Re: 79' Rs rear disc brake
Posted: Sun Apr 22, 2018 5:41 pm
by Zombie Master
If you pads haven't been contaminated, I would just service the system. Is the movement of the piston in some way restricted perhaps by the guide pin that is corroded, and needs cleaning and lubrication? A full service fluid change and bleeding should answer your question. It is not unusual that the rear brake only provides only 20 to 30% of the braking.
Re: 79' Rs rear disc brake
Posted: Sun Apr 22, 2018 5:44 pm
by Kurt in S.A.
Generally, any rear brake is going to be of limited use due to weight transfer. You say you have good pedal, but have you been successful bleeding the rear caliper? I understand that it has to be removed and rotated so the bleed nipple is at the high point. If you've done that, then stand by for mo' better input. But probably, you got what you got.
Kurt in S.A.
Re: 79' Rs rear disc brake
Posted: Sun Apr 22, 2018 7:59 pm
by SteveD
Rob Frankham @BMBikes airheads forum wrote:
These brakes are tricky blighters at best but they can be made to work satisfactorily (up to a point) with a bit of TLC. Here are a few points to look at...
Bleeding - The position of the caliper makes it impossible to bleed the system as it stands. The bleed nipple is lower than the cylinders on the caliper so any air trapped in the top of the cylinders will not be removed by bleeding no matter what type of bleeding equipment you use. The trick is to remove the caliper from the bike (leaving tyhe pipe connected) and support it with the bleed nipple at the top then bleed as normal. You will save yourself a lot of cursing and heartache if you put something between the pads to stop them being pressed together. I use a discarded ceramic tile but anything of suitable thickness will do.
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pushrod adjustment - The linkage between the top of the lever and the master cylinder is adjustable. Shortening it will raise the 'bite point' of the brakes. This is an adjustment that is often neglected and can have a profound effect on the useability of the brake.
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Pedal stop adjustment - At the very top of the pedal, there is a short bolt. The head activates the rear brake light switch but that isn't it's main purpose. It is a stop that sets the rest height of the brake pedal.
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Pedal pivot - The pedal pivot itself is a source of problems in this brake set up (this also applies to drum braked models). The pedal 'should' be bolted rigidly to the pivot bush which should rotate freely in the housing welded to the frame. Un fortunately, the whole assembly is vulnerable to water and road dirt and the bush tends to seize in the housing. When this happens, lever starts to move on the bolt and becomes loose. The most obvious symptom of this is when the lever is able to move sideways by a substantial amount but it also reduces the effectiveness of the lever.
...Attention... will help by ensuring that the brake is as effective as possible and, in my experience, it is possible to have a perfectly effective rear brake without resorting to modifications.
The rear can be made to slow the bike but really it's only useful in a few circumstances.
1. Trail braking in dirt, especially downhill.
2. Braking before corners, but more for steadying the rear and improving stability. The front does most of the stopping.
3. Slowing down in traffic, eg towards traffic lights.
I doubt I'd ride mine if I had to rely just on the rear brake.
Re: 79' Rs rear disc brake
Posted: Sun Apr 22, 2018 8:09 pm
by bluesman
I have not bled system the with the caliper off. I will try that, but my experience has been if there is air in the system it usually causes a spongy pedal.
Bluesman
Re: 79' Rs rear disc brake
Posted: Sun Apr 22, 2018 11:34 pm
by SteveD
bluesman wrote: ↑Sun Apr 22, 2018 8:09 pm
I have not bled system the with the caliper off. I will try that, but my experience has been if there is air in the system it usually causes a spongy pedal.
Bluesman
Yup. I use a syringe and push the old fluid back to the reservoir, then remove it, then flush new fluid in from the caliper.
It might also help flush small debris up and out too???
Maybe the cylinder is mildly corroded and needs a look see?
Re: 79' Rs rear disc brake
Posted: Mon Apr 23, 2018 2:53 pm
by bluesman
Thanks for info and input. I will try bleeding the caliper the way you described. One question is do I have to remove the rear wheel to remove the caliper? Looking at my Clymer manual I think it say's to partially remove the axle?
Re: 79' Rs rear disc brake
Posted: Tue Apr 24, 2018 12:43 am
by SteveD
The caliper sits atop that bracket that the axle goes thru. Loosening it will mobilize it easier. The limitation will be the brake hose which hasn't got much slack and the support arm which you can disconnect easily enough.
If you do remove the wheel, the advantage is you get to clean and reapply fresh spline lube 
Re: 79' Rs rear disc brake
Posted: Tue Apr 24, 2018 11:17 am
by Gibson
I had an 80's rs. Best looking bike in the world, great engine, but the brakes were barely adequate. I think a master cylinder with a smaller dia. cylinder to improve ratio for more mechanical advantage is the solution to the brakes that may be the least costly.
Re: 79' Rs rear disc brake
Posted: Tue May 01, 2018 10:24 pm
by Wobbly
I have the same bike and brake. I have to agree with all that was quoted from Rob. I'd also like to add....
► The secret to bleeding the brake is to lift the caliper off its mounting plate so that the nipple is the highest point in the system (just as Rob said). The shortcut to removing the caliper is to shorten the bottom mounting bolt by 2mm. In this way the bolt comes free of the mounting plate despite the close proximity of the swing arm.
► If you do not know the age of the brake pads you are ALWAYS better off to replace them. I like the ones from EBC better than the BMW versions.
► Braking effectiveness is always improved by substituting modern "stainless steel" brake hoses for the rubber OEM versions. This is true for both front AND rear hydraulic brakes. Not only are steel sheath hoses more effective, but the OEM hoses are known to fail internally and lock up the brake. So you gain braking power, while dodging a nasty bullet.
► Owing to the amount of water the rear m/c is exposed to, it is imperative that all the brake fluid be replaced on an annual basis.
Hope this helps.