If you like our site, please consider joining our club!
By joining you will help ensure that we can continue to provide this service
JOIN HERE!

R90/6 Brakes front disc brakes

Post Reply
redavide
Posts: 108
Joined: Tue Dec 04, 2007 12:33 pm

R90/6 Brakes front disc brakes

Post by redavide »

My '76 R/90 has dual front disc brakes. Late last summer, thanks to help from people on this forum, I was able to rebuild the calipers, etc. and then put about 1000 miles on it with the brakes working really well. It's been sitting in the garage for the past 4 months without being used and now I just noticed that the front brakes are locked - not completely, but the front wheel is really hard to turn. The brake lever feels like there's no play in it at all. Any suggestions as to what steps I should take to hopefully resolve this issue?? Thanks.

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

Re: R90/6 Brakes front disc brakes

Post by schrader7032 »

Try cracking the bleeder on either side or both...just enough to relieve the pressure. You might see some fluid come out so be ready to catch that or prevent it from going all over. If that helps, then it suggests that the return hole in the master cylinder under the tank is plugged not allowing the fluid to return.

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.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

Carott
Posts: 64
Joined: Tue Apr 14, 2015 9:18 am

Re: R90/6 Brakes front disc brakes

Post by Carott »

Redavide,
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.

chrishea
Posts: 68
Joined: Tue Aug 04, 2020 12:54 pm
Been thanked: 3 times

Re: R90/6 Brakes front disc brakes

Post by chrishea »

Do you have the under tank master cylinder? You mentioned no play, have you tried to loosen the cable from the brake lever to the master cylinder? Have you ever rebuilt the master cylinder? It may be going bad and the return port is clogged.
Chris
Seattle

redavide
Posts: 108
Joined: Tue Dec 04, 2007 12:33 pm

Re: R90/6 Brakes front disc brakes

Post by redavide »

chrishea wrote:
Wed Feb 23, 2022 11:10 pm
Do you have the under tank master cylinder? You mentioned no play, have you tried to loosen the cable from the brake lever to the master cylinder? Have you ever rebuilt the master cylinder? It may be going bad and the return port is clogged.
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 . . .

User avatar
San Arthur
Posts: 577
Joined: Sun Apr 22, 2007 2:08 pm
Location: San Antonio, TX.
Has thanked: 1 time
Contact:

Re: R90/6 Brakes front disc brakes

Post by San Arthur »

When left too long, moisture can find its way into the caliper's piston and cause the piston to erode – leading to the seizing.
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?
San Arthur in San Antonio, Texas
'58 R26 '76 R90S '88 R100RS '94 R100GS/PD

redavide
Posts: 108
Joined: Tue Dec 04, 2007 12:33 pm

Re: R90/6 Brakes front disc brakes

Post by redavide »

Thanks for the replies. As it turns out, I had wrench in hand prepared to open the bleeders, but before doing that I was squeezing the brake lever in and out while trying to move the wheel by hand. Suddenly, the brake lever loosened up and the wheel became free and the brakes seem to work normally.

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 . . . .

User avatar
srankin
Posts: 1054
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA
Been thanked: 14 times

Re: R90/6 Brakes front disc brakes

Post by srankin »

Thanks for the follow up and a note on your fix, Good luck and riding, St.
Owner of a 84, R80RT and 78, R100RS

kevindr
Posts: 5
Joined: Fri May 20, 2022 2:17 pm

Re: R90/6 Brakes front disc brakes

Post by kevindr »

Perhaps one of you can assist with my conversion to dual front on my 75 R90/6

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.

Post Reply