Rear brakes not working

[3dachshunds]

I am still working on new to me 1992 K75. There is something wrong with rear brakes. One morning I noticed rear wheel was seized - it would not turn with bike it neutral. Then it un-seized on its own, but here is another problem: the rear brake has no stopping power at all, it does not slow down bike at all when it is in motion. When I put the bike on a center stand, spin the wheel and step on the brake pedal, it does stop the wheel. But again, it does nothing when riding down the road. There is plenty of material on brake pads. I am thinking it is either bad caliper or master cylinder. How should I diagnose this? Many thanks!

[Laitch]

Remove the brake line from the caliper to determine if its clogged with old fluid and rotted hose particles, like these parts.

[Chaos]

If it's not clogged lines I would suspect a frozen caliper piston.  Easy to check if you pull the caliper off.

[3dachshunds]

So while trying to figure out why my rear brakes not working, I found that brake light stays always on. It appears that brake pedal is sagging, causing micrswitch contact stay open.
IMG_3519.jpeg (79.71 kB . 768x576 - viewed 347 times)If I manually lift the pedal up, it closes swith and brakes light goes off, and reacts properly to front brakes. It feels like there is missing return spring tension that would keep brake pedal up and pressing against switch. Also, there is very short stroke in master cylinder, how can one tell if it works at all? Many thanks!

[Laitch]

Were the fittings and brake line clogged, or what? If the line is clogged or the cylinder bore is filthy, the master cylinder piston will have trouble rebounding after being compressed. Cylinders in good shape will allow the piston to rebound with the master piston spring alone, but the pedal retraction spring makes it quicker.

The bracket is mounted to the piston using the adjusting screw (12 in the diagram); the nut on the screw not only works with the screw to adjust the piston stroke but also secures the bracket (8 in the diagram) to the pedal. The retraction spring (9 in the diagram) hooks into a hole on the bracket and hooks around a pin on the stand assembly that also anchors the stand springs.





Start using MaxBMW's parts fiche to help in identifying the parts of various assemblies you encounter.

[3dachshunds]

Please correct if I am wrong, but by looking at Max BMW fiche, it appears there must be L shaped bracket attached to master cylinder, where return spring should be attached (circled). I I don't see how to attach L bracket; should I put the bracket on the existing thread and screw a nut on top of it?

[Duckbubbles]

YES.  It goes between the pedal boss and the nut on the master cylinder piston extension.

Frank

[Laitch]

I don't see how to attach L bracket; should I put the bracket on the existing thread and screw a nut on top of it?

The parts fiche is created to describe parts in an assembly; it isn't an assembly tutorial. Clearly another photo is necessary. Here's one from the Internet. Notice that the short leg of the bracket will receive the spring hook; the long leg of the bracket is attached to the pedal with screw and nut assembly. Keep in mind, a good adjustment of that screw-and-nut assembly can only be made when the master cylinder is in good condition, the pedal is in its fully retracted position and the cylinder piston is in its fully rebounded position.

[3dachshunds]

Thank you! I think I need to order the spring and L bracket, probably master cylinder overhaul kit as well.

[3dachshunds]

Were the fittings and brake line clogged, or what?

I have yet to disconnect and see if it clogged, probably will replace brake line, as I don’t know if it was ever replaced

[Laitch]

If you buy an OEM pedal spring, you'll note the hooks on the ends differ from each other. Mine is set up with the V- bend on the left end in the photo hooked onto the pedal bracket and the rounded end on the right hooked around the stand pin.

[mmc_meneses]

Hello everyone from Spain,

I would strongly recommend you to change the brake hose for a metal one, since i did it everything changes, sensations at braking, accuracy... strange thing is now it looks like sequencial (I now the brake pump in our flying bricks is nothing electronical), the faster u go the closer is the pedal to break, slower you have to go deeper.

Also in the process of changing it I discovered that the original rubber one is like closing around itself everytime you release the presion on the pedal so if the bleeder is open in the process the air don't go back to the line... so is easier to purge... BUT... when the bleeder is closed as in riding conditions sometimes (temperature, brake pads condition...) it doesn't send back pressure to the pump and the braking is much less accurate (even like in my case it doesn't work anymore until the pressure come back to the pump).

But with the metal hose, apart of being quite complicated to purge, the pressure through the line flows perfect, doesn't matter the temperature, and the pump is always ready.

Long story short, changing the hose to metal for me change the whole driving experience

[The Mighty Gryphon]

Hey!  Welcome mmc_meneses! 

[3dachshunds]

All right, I put my rear brakes on hold until rear MC rebuild kit arrived today and it seems I hit a snag. I am hoping you advise me. Let me tell you what i did so far: removed brake line and MC plunger, thoroughly cleaned the bore and inspected brake line for gunk (there is no banjo bolt on caliper side). Everything seems squeaky clean. Here is the problem: When I insert new plunger (it was soaked in brake fluid before pushing in) I am able to pump it with my thumb and observe brake fluid squirting from caliper bleed valve. However, when plunger is pushed all the way in, it becomes stuck in the bore. I have to forcefully free it. It pumps and glides smoothly in the bore, but gentle push to bottom position locks it… So here I am folks. Your help much appreciated!

[Laitch]

The piston is going in as far as you're pushing it because it isn't meeting the resistance caused by the pushing of a full hose-worth of fluid against brake pistons that only have a few millimeters to move anyway. Stop doing that even though its fun.    The proof will be what happens when the pistons and pads are in place around the disc. If you've inspected the bore and it's in good condition, move on.

[3dachshunds]

Thank you L, I am thinking to apply continuous negative pressure to bleeding nipple to make replace trapped air with brake fluid. I was unable to find proper size return spring for pedal, the one from home depot too thick and rubs against MC boot and frame. Probably have to order from Max BMW

[Laitch]

Does your Brick have ABS?

[3dachshunds]

Mine is non-ABS.

[Laitch]

I have Speed Bleeders on calipers both front and rear—SV1010S. I flush the brake systems twice a year. Those bleeders simplify the job for me and are durable so they encourage regular maintenance.

[3dachshunds]

Update: Finally have working rear brakes! I bought a compressor driven power bleeding kit from Amazon and it did a trick. I have bled automotive brakes many times without any vacuum, with one person pumping pedal and another working bleeder screw.   

[blackie1]

thanks for that link Laitch.