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Topic-icon 1975 R90/6 Front Brakes

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2 months 2 weeks ago #4098 by 14069
1975 R90/6 Front Brakes was created by 14069
I have owned a 1975 R90/6 for about 7 years with current mileage at 52,797 miles and have a question about the front brake. The bike was completely re-worked mechanically and looks and runs great. My only issue is with the front brake which basically is worthless in stopping the bike. This is the original front braking system, part mechanical and part hydraulic. The brake line to the master cylinder, master cylinder, brake line to the caliper, caliper and pads were all replaced with new parts and the disk is in good shape. Since I have had the bike the front brakes have been poor. Even with the brake applied from a stop you can push the bike with minimal effort. You can tell the brakes are on but barely. I don't know if this is all the single disk should provide or if I should look for another problem. I have replaced the brake pads twice and sanded the disk to remove a little glazing but that has not improved the performance. The brake line is not stainless steel which is the last change I know of to make the front brakes perform better. The rear mechanical brake substantially out performs the front disk. Any suggestions will be appreciated.

Sid - Clover, SC

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2 months 2 weeks ago #4100 by dcb_wvu
Replied by dcb_wvu on topic 1975 R90/6 Front Brakes
I am looking at a similar bike and the buy is already trying to fit a /5 front wheel on it. something went wrong with the master cylinder. My solution upon getting the bike is to install the master cylinder on the handlebar and then stainless lines to the caliper in one run. Stainless lines will surely help but I have talked to many owners who are doing what I am planning and recommended it too.

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2 months 2 weeks ago #4102 by 14069
Replied by 14069 on topic 1975 R90/6 Front Brakes
I knew that was an option but trying to keep the bike original. Thanks for the reply and good luck with you bike.

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2 months 2 weeks ago #4103 by dcb_wvu
Replied by dcb_wvu on topic 1975 R90/6 Front Brakes
I can appreciate the originality approach but I would consider all the upgrades along the way that still do that in spirit, so the SS hoses would be a good option. You can also rebuild the calipers, master cylinder, etc. almost all of the moving parts. Good luck and remember to change/flush with new fluid annually.

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2 months 2 weeks ago - 2 months 2 weeks ago #4104 by jagarra
Replied by jagarra on topic 1975 R90/6 Front Brakes
Your bike should have a 14mm M/C with a 40mm caliper. I had the same combination and the braking was poor, the lever took a lot of pressure and it wasn't responsive enough to variations of pressure, felt wooden to me. You indicate that you seem to be losing the grip the pads have to the disc, this could be due to your original rubber lines deteriorating internally restricting flow. Best to get a Speigler replacement.
When the discs were introduced the BMW engineers were afraid of front wheel lock ups so the ratios were keep high, so feel was sacrificed as a result. In order to obtain the proper ratios one has to either increase the caliper piston diameter or decrease the M/C piston size, I am afraid that you are already at those levels based on the rebuild kits available from BMW. the only other option if the new line doesn't change your feel is to go to a handlebar master or go to dual discs which is what I did to solve the braking issue.
Last edit: 2 months 2 weeks ago by jagarra.
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2 months 1 week ago #4106 by Wobbly
Replied by Wobbly on topic 1975 R90/6 Front Brakes

jagarra wrote: When the discs were introduced the BMW engineers were afraid of front wheel lock ups so the ratios were keep high, so feel was sacrificed as a result. In order to obtain the proper ratios one has to either increase the caliper piston diameter or decrease the M/C piston size, I am afraid that you are already at those levels based on the rebuild kits available from BMW. The only other option if the new line doesn't change your feel is to go to a handlebar master or go to dual discs which is what I did to solve the braking issue.


I don't think that the handlebar m/c is the ONLY option. At one time "sleeving" the m/c you already have down several MM's was an option. I've read about it here, but don't have a name of anyone to give you.

These systems were never able to do "stoppies", but a VERY hard look at EVERY system component will yield excellent results. The devil is in the details.

Several ideas....
• Of all the replacements available for the stock system, "Steel" brake lines will help the most. Speigler is a really good brand, but there are many others too. Bob's BMW offers them on their web page.

• When DOT4 brake fluid gets to be over 5 years old, it can "grow" all sorts of varnishes and crystalline growths on the inside of your system. These can severely limit the mechanical movement of m/c and caliper pistons. It's not clear from your report if both units have been disassembled, thoroughly cleaned, inspected and reassembled. Simply installing new seals won't do the job.

• Likewise, the mechanical portion of your brake system has to be reworked to an equal level. Have you replaced the cable ? Are both levers free to move over their entire arc of action ? Have you lubricated both pivot points with water-proof grease ? (IMHO a worn-out cable can add as much "sponginess" as the rubber brake lines.)

• Brake compounds have changes in the last 40 years. You said you had new pads.... but you didn't say from whom and when those pads were made. I like the new pads from EBC.

• We're talking about the ATE system, which has an adjustment for the caliper location on the bottom of the fork leg. No mention of that in your report.

• You most definitely want to investigate the EBC "floating disc" available for abut $200. The newer disc material coupled with matched pads from EBC will double your braking power. The difference is amazing.

• Lastly, how you bleed these systems is MASSIVELY important. If you can roll the bike with the brakes applied,, then I can almost guarantee you that there is still air in the system. Note that the frame mounted m/c is tilted the WRONG direction for "self bleeding". Try installing brake fluid from the caliper AND bleeding while the front wheel is 6-10" lower than the rear.


Those ideas ought to get you closer to happy.

Don't hide 'em, Ride 'em !!
#15150

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