1990 K75 Rear Wheel Lockup

[Fortunato123]

Today, at 80 mph my 1990 K75 came to a screeching halt when the rear brake locked up suddenly. It was very dangerous and I almost got rear ended. I managed to pry the caliper from the disk with the toolkit screwdriver to get the bike going again.

I appreciate all the helpful posts and advice. I have stainless steel brake lines, but will take a look at the master cylinder. Is there a way to test it on the bike or on the bench to be sure this won't happen again? The back brake was working fine when I took started on my journey and it seems to work ok again now. I am concerned that I may not survive another surprise reoccurrence Thanks again!

 

[98lee]

Fortunado,

Most likely, the problem is the pedal return spring is broken or not connected causing the rear brake to drag or there is no freeplay adjusted between the pedal and the master cylinder.

How’s the vacuum leak on your injectors? Please update your other thread.

 

[69ZEFF65]

Check your master cylinder to ensure the fluid return hole is not plugged. If it is the pads would stay engaged to the rotor, pressure will build quickly and the brake will lock up. If the master cylinder has never been rebuilt it would be a good time to do it.

Also you don't say if you have ABS on the bike, the module could have a blockage in there that prevents the fluid return also.

 

[robsryder]

I had a rear wheel lockup on my 92 K75S whilst going 60 mph down a freeway, in a construction zone (meaning there was a concrete barrier immediately to my right; I was in the right side lane). I was packed up heading from NC to the top of Michigan for an RA rally. Suddenly the rear tire locked up and burning rubber smoke was billowing out the back. Cars behind me screeched to a halt. Nowhere to pull off and I couldn't move the bike backwards or forwards.

I apologized to the car behind me and asked if he could put on his emergency flashers and call 911; I was going to try for a tow truck. Fortunately there was a tow truck right behind the car, although he was not set up for bikes. He did, however, put on his flashing lights. Naturally this was early morning and traffic was really piling up.

I still couldn't move the bike, but the tow-truck driver made a discovery. My rain suit (frogg-toggs with aerostich lobster-claw glove covers) in its bag was tightly wedged between the rear tire and swingarm. The tow-truck driver helped me dislodge the now shredded rain suit. The bike started and I got on and was able to ride down the road.

I was still quite shaken up and took the next exit. I stopped at a nearby grocery store parking lot. I tossed the shredded rain gear in the trash. I opted to return home (I was about 45 minutes into my trip). I was a bit concerned about the rear tire having lost so much rubber from one spot. Once home I transferred my gear to my R1100RS and went to the RA rally.

The problem, I think, was me being distracted while I was attaching the rain gear bag to the bike. This was about my last activity and I was talking to my wife and not focusing on securely attaching the bag. Vibrations and wind likely caused the bag to become unstable and rather than fall from the bike and land in a ditch, never to be seen again, got caught in the wheel and stuck thereby locking up the rear tire.

When I returned from the RA rally I removed the 92 K75S rear wheel and installed a new tire. It was very evident where the tire skidded to a stop from 60 mph.

The moral of the story is to be really careful in securing your load!!

 

[98lee]

The pedal return spring attaches inbetween the two centerstand retract springs at the front end and to a tab on the pedal to master adjustment screw on the rear.

 

[PGlaves]

A general comment about brake lock-up. If for some reason the pads fail to retract and continue to rub the rotor the caliper heats up and the fluid in the caliper expands. If the fluid is trapped then it gets hotter and expands more until the brake locks up. If you are riding somewhere where you are accelerating and decelerating you may feel the drag before the wheel locks up. On a steady drone you may not feel the drag so soon.

If a brake does lock up as described by the OP the easiest immediate fix to be able to roll the bike is WITH PROPER PRECAUTIONS SO AS TO NOT BECOME BURNED to open the bleed screw to relieve the pressure.

 

[kbasa]

A general comment about brake lock-up. If for some reason the pads fail to retract and continue to rub the rotor the caliper heats up and the fluid in the caliper expands. If the fluid is trapped then it gets hotter and expands more until the brake locks up. If you are riding somewhere where you are accelerating and decelerating you may feel the drag before the wheel locks up. On a steady drone you may not feel the drag so soon.

If a brake does lock up as described by the OP the easiest immediate fix to be able to roll the bike is to WITH PROPER PRECAUTIONS SO AS TO NOT BECOME BURNED is to open the bleed screw to relieve the pressure.

In my experience, this kind of failure is usually due to the master cylinder not allowing fluid to return from the caliper, so the problem isn't with the caliper at all, but the MC. I had a CB750 Honda that did that. I rebuilt the calipers, but nothing changed until I disassembled the MC, cleaned it and rebuilt it. It had collected some crud in the return hole and once cleared, that was the end of that issue.

 

[Fortunato123]

Thank you all for your suggestions. You were right. It looks like there was no freeplay between the pedal and the master cylinder. I backed it off and took it for a ride (even on the freeway) where I pretty much only used the back brake and I had no problems with it. No hint of a lockup at all. Yea!

I came back and just wanted to bleed the caliper because the pedal felt a bit spongy. I put a hose on the bleeder and opened it. When I pressed on the pedal, a tiny bit of fluid came out. I closed the bleeder and let the pedal up, but that was the last time fluid would come out. I tried just closing the bleeder and to pumping it back up, but no luck. It did not seem to be taking brake fluid from the reservoir despite the tube being free of kinks. When I took the boot off the MC, the piston did not appear to be returning toward the pedal. Any suggestions would be appreciated. It was working and now it is not!

Thanks again!

 

[98lee]

Thank you all for your suggestions. You were right. It looks like there was no freeplay between the pedal and the master cylinder. I backed it off and took it for a ride (even on the freeway) where I pretty much only used the back brake and I had no problems with it. No hint of a lockup at all. Yea! I came back and just wanted to bleed the caliper because the pedal felt a bit spongy. I put a hose on the bleeder and opened it. When I pressed on the pedal, a tiny bit of fluid came out. I closed the bleeder and let the pedal up, but that was the last time fluid would come out. I tried just closing the bleeder and to pumping it back up, but no luck. It did not seem to be taking brake fluid from the reservoir despite the tube being free of kinks. When I took the boot off the MC, the piston did not appear to be returning toward the pedal. Any suggestions would be appreciated. It was working and now it is not! Thanks again!

Fortunado,

So, you are saying that with the pedal fully pulled up and the adjustable rod between the pedal and the master cylinder fully against the pedal and a gap between that rod and the master cylinder piston, the piston does not fully extend to the snap ring inside the master cylinder?

If that is the case, either the master cylinder is gunked up (official BMW term from the German word
Gerunkeded) or the return spring inside the master is broken or missing. Sounds like it’s time to remove and disassemble the master cylinder to find the cause.

Remember, these master cylinders are no longer available, so don’t do any additional damage to the cylinder or the seals!! Look for any gunk in the cylinder or a ridge that might cause it not to return.

With the piston out of the cylinder, blow compressed air through the fitting from the reserviour into the cylinder. Does it dislodge any gunk into the cylinder?

 

[Fortunato123]

Thanks Lee, I appreciate the prompt reply. I'll start the disassembly to see if I can find the problem. It is disappointing because it was working so well after I made the adjustment and before I tried to bleed it. Thanks again!

 

[Fortunato123]

I took out and disassembled the MC today. The piston did not want to come out of the bore. I had to put it in a vice to get it out and when I did the metal shavings and the scored bore told a very bad story. I had another 13mm MC from a bike that we salvaged the block from, but in disassembling it we found the bore scored in there as well.

It appears that the MC for this bike is no longer available and extraordinarily expensive if you can find them. I've seen some posts on a generic replacement. What is the latest thinking on that option - or any other options? At this point I am without a bike.

I just saw the post re the 16mm replacement. Is that the best/recommeded option at this point? https://www.euromotoelectrics.com/product-p/bmcylinder142u.htm

Thanks again!

 

[98lee]

I bought a couple of the 16mm masters to give them a try. Haven’t had the time to swap one out. In theory, it should be ok, but that's just a guess. Last time I talked to EME, they only had a few left and weren’t sure if they would be getting any more. You can thank the accountants at BMW for not keeping the originals available.

 

[Fortunato123]

Thanks Lee, I just ordered one. I'll let you know how it goes when I get it.

 

[98lee]

The master cylinder does not come with the pushrod between the pedal and the master cylinder piston. You need to use the one from your old master.

Because of the different diameter of the inside of the new piston, it looks like you need to make a bushing to keep your old pushrod centered in the new master. I’m guessing it can be made from any firm material (pvc, nylon, aluminum, etc.)

 

[omega man]

This place seems to have good from forum posts-

Automotive hydraulic specialists since 1973

The worlds largest rebuilder of lever action shock absorbers. Apple Hydraulics has served classic and vintage car enthusiasts since 1973 BUICK, FORD and MORE

www.applehydraulicsonline.com

OM

 

[98lee]

This place seems to have good from forum posts-

Automotive hydraulic specialists since 1973

The worlds largest rebuilder of lever action shock absorbers. Apple Hydraulics has served classic and vintage car enthusiasts since 1973 BUICK, FORD and MORE

www.applehydraulicsonline.com

OM

From the other current thread on the rear master cylinder NLA situation:

“Brian Kohn​

New member​

• Feb 17, 2024
• 
  
• Add bookmark
• #15

Thank you Frank & Lee for all the info . I will replace the hoses. I spoke with Apple hydraulic they wiinot rebuild the Magura master because the inlet plug is impossible to remove without breaking . He did say some of the rebuild kits he was found have the wrong size piston causing this problem. I will try to get a rebuild kit from him. Very helpful, knowledgeable, & willing to share. Thanks again for all the feedback!“

 

[omega man]

This is the thread I found Apple Hydraulics on-

Shout Out - Apple Hydraulics

I had Apple Hydraulics re-sleeve the original master cylinder on my '74 R90s and it came out great. With 150k, my last rebuild (new piston and seal) was still allowing a drip or two of brake fluid in the rubber catch boot, so I took advantage of their re-sleeving service. Got it back, bench...

forums.bmwmoa.org

In hunting down critical parts, I keep hunting.
OM

 

[Fortunato123]

Lee, Was my update post deleted?

 

[98lee]

Lee, Was my update post deleted?

According to your posting records, you haven’t posted since your three posts on Dec. 15.

 

[Fortunato123]

Thanks Lee, That is strange. I will attempt to recreate my post below.

Greetings and Happy Holidays. Lee, thank you for the tip about the sleeve needed on the 16mm MC. My MC reservoir hose was also blocked, so I bought some 6mm ID Tygon hose to replace it.

The Tygon was also a great sleeve for the MC pushrod. I just heated it up a bit, oiled the pushrod, pushed it into the hose and cut it off at the end. Almost a perfect fit. I then used a dab of hot glue at the end of the pushrod to stick in the bottom of the hole. See photos below.

I am now in the process of rebuilding the rear caliper. The pads and seals got a bit too crispy for comfort after the real wheel seized at 80 mph.