R90S sticking front brake

[2slowwheels]

1974 R90S
The front brake is sticking. Is it most likely in the calipers or master cylinder? I've never had the old ATE calipers apart. Thanks

 

[kbasa]

If you're applying the brakes and they don't release, the return passage for the brake fluid in the MC is probably plugged. Been a minute since the fluid's been changed? They go together.

If you crack the bleeder at the caliper and let a smidge of fluid out, does it release the brakes? I'd start there.

It could be a collapsed brake line if they're the originals and are now 51 years old. It could be that the passages in the MC are plugged with crud. It could be that the pistons have rusted and are hanging up on the seals and can't retract.

Given that this is a 50yo motorcycle, I'd just disassemble the entire thing, replace the lines, rebuild the MC and calipers and maybe replace the discs with floaters from EBC. That braking system has had its day and doesn't owe you a thing.

OBSlow: my plate on my R100 is SLOWBKE.

 

[20774]

Yes!! Well, the first place I'd look is into the master cylinder under the tank. When you squeeze the handle, it pushes fluid out to the calipers. Then when the o-ring in the calipers push the pistons back, the fluid has to return to the master cylinder. There is a small hole in the function of the master cylinder that lets that return. Try cracking the bleed valve on the calipers...not enough to let air in but see if there is some fluid that wants to come out. That would be the fluid not being able to return.

Another thing...what about the brake hoses? Still original rubber? Could be they are breaking down and when the brakes are used, the hoses balloon a bit. That ballooning doesn't let the fluid return necessarily.

Beyond that, you might try a brake bleed and see if full function returns. Clearly stick brake is not good...could spit you out on the ground or damage the rotors.

Brook Reams is going to have some great writing and photos on his website for brakes.

BMW Motorcycle Projects

I’ve collected information that should help someone who is interested in doing rebuilds on older BMW motorcycles. The material is organized by each particular project. Links to write-ups abou…

brook.reams.me

Also Boxer2Valve has a couple of videos on front brakes...here's Part 1:

https://www.boxer2valve.com/r906-20-part-1-brakes-front-wheel.html

 

[ccolwell]

Yes!! Well, the first place I'd look is into the master cylinder under the tank. When you squeeze the handle, it pushes fluid out to the calipers. Then when the o-ring in the calipers push the pistons back, the fluid has to return to the master cylinder. There is a small hole in the function of the master cylinder that lets that return. Try cracking the bleed valve on the calipers...not enough to let air in but see if there is some fluid that wants to come out. That would be the fluid not being able to return.

Ditto. If you decide to rebuild the system, I'd start with the MC and the brake lines. They are easier to do than the the calipers. Some red rubber grease is essential.

If you do move on to the calipers, dismantle them before buying a kit. I made the mistake of ordering new pistons assuming they'd be bad on a 30 yr old bike. Not so much.