The Airheads Beemer Club is a non-profit club reclaiming the 'Legendary Motorcycles of Germany'

Topic-icon '78 R80/7 Front Brake (or lack thereof)

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2 weeks 1 day ago - 2 weeks 8 hours ago #6576 by dartnmartn
'78 R80/7 Front Brake (or lack thereof) was created by dartnmartn
Hello. New to the club.
I have a 78' R80/7. Bought it new back when I still had hair. Parked it against the garage wall in 2005 with 70K miles on it. Recently started it's resurrection (hopefully)
Pulling it to the center of the shop revealed zero front brake. So....
New caliper Piston kit, brake pads, new brake line, master cylinder rebuild kit.
Honed out the Master cylinder, put it all together and started the bleeding process. Mity Vac, old school lever pumping, an Exxon Valdez quantity of brake fluid thru the system... The lever still comes back to the bar. It doesn't come back easy, but it never gets "Solid" like my Japanese race bikes do. (not really expecting that but was expecting something close to the brakes I remember way back when.) A ride around the block revealed a gentle slow down was all it was capable of. No stoppies on this ride just yet....

So.... I sent the M'cylinder off for a re-sleeve. Reassemble. Same results.

Is there still air in there somewhere? I see ZERO bubbles exiting while bleeding. I tipped the M'cylinder up thinking there was air in the switch. There are no leaks.

I am stumped!

Please help get me back on the road to Airhead Riding. I have several Airhead friends and feel wrong showing up on a ride with 4 cylinders and 130 hp.

Thanks for ANY suggestions you may have.

Years ago I had a similar problem on a bike equipped with "modern brakes". I had aftermarket steel brake lines. Brakes started feeling mushy. Tried to bleed and no change. Rebuilt the master cylinder, no change. Put some more betterer 4 piston calipers on it and used the donor bike master cylinder. No change! I had re-used the steel brake lines. Swapped out to the donor bike OEM rubber lines.... BOOM. stiff brake lever....???? no leaks, no air bubbles. New steel lines and all was well.

Is this new rubber OEM style BMW brake line the culprit?

Brad
Last edit: 2 weeks 8 hours ago by Wobbly. Reason: Mods altered title

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2 weeks 7 hours ago - 2 weeks 7 hours ago #6580 by Wobbly
Replied by Wobbly on topic '78 R80/7 Front Brake (or lack thereof)
Brad -
First of all... Welcome Aboard !

Air inside your hydraulic brake lines is the issue.

First of all, Jap bikes follow a very precise brake hose layout that naturally results in that "hard" brake lever. This design never includes loops in the brake line or hose layouts where the master cylinder is not the highest portion of the hydraulic system. The result of this is that air naturally travels upward through the plumbing and naturally accumulates at the master cylinder, where it is allowed to "self bleed" its way into the m/c reservoir. Therefore, no matter how mentally challenged the owner might be, all he has to do is pour brake fluid into the reservoir and then start pumping the lever. Air wants to leave these systems by design.

However, the Germans have slightly different ideas. (Maybe you've noticed ?!)

1978 model probably has the ATE front caliper with the under-the-fuel tank master cylinder.
• First of all, the loop of flex hose coming off the fork leg may go slightly downhill as it makes its way to the frame attach point. You can't have that or air will accumulate there inside the line. Most of the time, merely pushing the hose to one side, or keeping it gently lifted, creates a gentle uphill flow over the full course.
• Secondly, the under-tank m/c is tilted slightly up hill. In it's factory position it can never fully bleed itself. So you'll either need to unfasten it from the frame or roll the rear tire up onto something about 4-6" higher.
• Thirdly, the German m/c can only self-bleed when the piston is allowed to fully return to the circlip stop. So if there is old brake fluid crud stopping that physical position from being obtained, then air can't come out of the system. This situation can also happen if you have your brake cable adjusted too tightly. The m/c piston MUST be 100% fully extended at rest.

If you'll do these 3 simple steps, the system will start to self bleed during 100 SLOW and STEADY pulls on the brake lever. With the cap REMOVED, watch the pool of brake fluid within the reservoir. You should see a progression of "Champaign bubbles" coming up in the reservoir. That is your problem air leaving the system and then being replaced with fluid. The more viscous the fluid the longer this process takes.

When the lever starts to get hard, you can also do a traditional bleed at the caliper to remove entrapped air within the caliper. Take your time. Operate the lever slowly, and it will get better.

► However, your brake lever will NEVER be as hard as your Jap bike lever for 2 reasons... You've got the extra built-in stretch of the front brake cable. You've got the expansion of the OEM rubber hose. I highly suggest you change over to "stainless steel brake hoses" as per THIS THREAD.


Hope this helps.

Don't hide 'em, Ride 'em !!
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Last edit: 2 weeks 7 hours ago by Wobbly.

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