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R90/6 Brakes front disc brakes
R90/6 Brakes front disc brakes
- schrader7032
- Posts: 9038
- Joined: Fri Oct 27, 2006 3:00 am
- Location: San Antonio, TX
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Re: R90/6 Brakes front disc brakes
Are the brake lines original...are they rubber? They could be failed internally in which they balloon up and don't deal with the pressure properly.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
Re: R90/6 Brakes front disc brakes
Short term solution just to be able to roll the bike around is to gently TAP on the caliper with a rubber mallet. This should allow you to move the bike.
A more in depth investigation is needed. You can confirm the master cylinder is working and the hoses are not plugged by cracking open the bleeders on each caliper.
If tapping on the calipers frees up the wheel and after braking it gets stiff again you may need to check the caliper pistons for peeling chrome and corrosion.
Good luck and stay safe.
Brett
Plus what schrader7032 said.
B.
Re: R90/6 Brakes front disc brakes
Seattle
Re: R90/6 Brakes front disc brakes
Yes, master cylinder is under the tank. I rebuilt the master cylinder and calipers last summer and everything was working fine afterward. Then it sat in the garage not being used for the last 3-4 months and this happened. I'll probably try what Schrader and Carott suggested and also take the tank off and adjust the master cylinder piston out . . . But I don't understand why this would happen during a time when the bike was just sitting there . . .
- San Arthur
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- Location: San Antonio, TX.
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Re: R90/6 Brakes front disc brakes
But only 4 months inside a garage? Not likely or at least I never had this problem before with my ATEs.
Is it possible for the master cylinder to pressurize the circuit by itself?
I think is more likely that the pistons never retracted from the last time you parked the bike in the garage.
Are both pistons fully extended?
I would start with the bleeders and see what happens? If pistons don't retract I would try removing the wheel and retract the piston by hand/tools.
If the problem is not down there, start inspecting up to master cylinder.
Brake fluid is corrosive and will eat paint, careful when working with the master under the tank, and good luck?
'58 R26 '76 R90S '88 R100RS '94 R100GS/PD
Re: R90/6 Brakes front disc brakes
I suspect one of the caliber pistons got hung up in the cylinder. When I rebuilt the calipers last summer, there were a few very small rust lesions on the pistons, but instead of replacing the pistons, I just polished the lesions with some emery cloth. Now I'm thinking that I should installed new pistons . . . . Anyway, probably after 4 months of sitting, the brake fluid probably drained out of the cylinders, causing the pistons to become unlubricated and when I start using the bike regularly this won't happen again . . . .
- srankin
- Posts: 1078
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Re: R90/6 Brakes front disc brakes
Re: R90/6 Brakes front disc brakes
I am converting my 75 90/6 to dual discs. Put everything together last night, with the left caliper going in nicely. When it came to the right side, the hole in the caliper would not line up with the holes in the slider. My first inclination is to shim behind the disc on the right side. Alternatively I could slightly increase the axle spacer, but that may throw off the left side.
I measured the space between the outside face of the disc, and the top "arm" on the slider at the closet point. Left side is 1.3mm and the right is 2.25mm. To shim out the right side will take one or possibly two washers, or bmw part # 34111232417.
All advice would be appreciated.