/7 brake woes -- frozen front brake

Decided to start prepping the bike for riding season after over 100" of snow this winter.
Bleeding the brakes (single disc front with under-the-tank frame-mounted master cylinder). For some reason, the brake lever remains soft, no resistance. Also, it feels like the front brake is on, b/c the front wheel does not spin freely, even though the brake lever is not pulled in. Known minor master brake cylinder leak.
Upon closer examination, pulling the brake lever in does not move the brake pads. They are frozen stuck, onto the rotor. That would explain the lack of free spinning front wheel.
What am I most likely dealing with here? I searched the forums for keywords "frozen", "brake", "stuck", "caliper" in several combinations and yielded unrelated results.
A push in the right direction would be obliged.
Thank you.

A few thoughts:

- blocked return hole in the master cylinder piston
- torn o-rings on the master cylinder piston
- internally damaged OEM rubber hose (any bulges along the line when you operated the lever?)
- corrosion inside the caliper not letting the piston move either way

What do you mean by "known minor master brake cylinder leak"? If you know there's a leak, you really should fix that first.

If the o-rings are torn on the master cylinder piston, then when you squeeze the lever, the fluid just simply goes back and forth past the o-rings instead of being shoved down to the caliper. That could be the source of the softness. Or you could still have air in the lines.

I take it the front wheel is not completely locked? First, pull the caliper and make sure something else isn't causing the friction on the wheel. I would keep open the possibility that there are two problems: one, air in the hydraulic system and two, a frozen caliper. Use a C clamp to push the piston back into the bore, then rebleed. If you still have no resistance at the lever, then something is wrong with the hydraulic system as you should get resistance with a frozen piston. Then you're looking at a rebuild.

It's time

Time to rebuild the brake system. Rebuild or replace the caliper, master cylinder, and replace the rubber brake lines. Anything less just won't be good enough. Those brakes are old. They lasted a long time and have served you well. Put them out to pasture. Good luck.

Ride Safe
Rideoften

O-ring

When I was changing the brake fluid in my R90/6, I found that the o-ring between the brake fluid reservoir and the master cylindar would no long keep the brake fluid/paint stripper in. This let air into the brake line. After replacing that o-ring and bleeding the system everything worked again.
Does releasing the brake lever not suck the pucks off the rotor - as long as there is no air?

What causes the caliper pistons to retract is the resiliency of the square cut rubber piston seal.

There is a veritable host of possible problems here. My guess is they fall into two camps; one storage, the other age related.

Storage: A sad thing to have to do to a bike (or any motor vehicle) they jusy don't respond well to prolonged dormant spells in the cold, unheated garages where most of us keep them.

Corrosion build-up on the caliper pistons is a classic cause of "frozen brakes" there isn't much drawing the pistons away from the disc other than the above mentioned tension from the seals and whatever lateral play there is in the disc itself. The solution is pull the caliper, clean it and replace anything suspect.

The pads themselves might also be stuck in the caliper, cleaning should solve that too.

Age related: What can I say? We all get old. Fortunately it's easier to replace bike parts than people parts. That soft brake lever could be any number of things from hoses to seals gone bad. Squeeze the brake lever while watching the hoses, do they swell? They should not. I was trying to figure out a soft pedal on my F650 and had a hose rupture while up on the lift, that bike was built in 2001.

You mentioned a slow leak, is it faster now? Is there a small jet of brake fluid shooting across the room? It's been a long time since I had to rebuild an under-tank master cylinder, assuming the kits are still available and your MC isn't shot from corrosion a rebuild isn't that hard.

All,
Thank you for the responses.
The "minor" leak of the master brake cylinder was noted at the end of last riding season, a rebuild kit is on its way. It seeps a very small bit of green fluid from the piston area when the piston is moving in and out. It has leaked onto the frame. It is not enthusiastically seeping or hemorrhaging.
No bulging of the old rubber lines was noted. However, they are cracked and old and should be replaced.
The front wheel is not completely locked. It can rotate with little effort, but it will only spin a little bit then stop. When dead rolling the bike, you hear the squeak from the pads being locked onto the rotor, and feel the slight resistance from that.
After bleeding the brakes until I saw no air bubbles coming out, the brake lever did build a little bit of resistance. That is, it is no longer completely limp soft, and the lever can no longer be pulled in all the way, 100%. However, it is far from the stiffness that it was last riding season. If I dead roll the bike, I can bring it to a stop with the brake lever, although it must be pulled as far in as possible, whereas "normally" it would only take about an inch in to stop the bike. Again, I don't see the brake pads move, but there is braking taking place when the lever is pulled in as far as possible, although it is weak and can be overpowered by simply pushing my weight back and forth onto the bike. Hope that makes sense.
The bike has sat through the rough Wisconsin winter outside, covered.
I will check the things you mentioned and report back.
Thanks again.

What you describe sounds about like my /7 was. If your caliper piston isn't corroded too badly, it can be reused, saving $$$. BMW Hucky in Florida sells a basic caliper rbuild kit (less piston).

Rebuild master cylinder, rebuild caliper, replace hose

like said, Huckys sells a minor kit for about half or less then a BMW caliper kit, and replace the hose with a stainlesss hose ( Speigler or other)

got the extra bucks, replace the front pads with EPE pads

do all that as listed above, and the bike will stop better then new

On my 76 R75/6, I rebuilt the caliper and replaced the old brake hose with a new BMW rubber hose, but results were not satisfactory. I recently cleaned and lubed both of the ends of the cable from the lever to the under tank mc, and replaced the new hose with steel braided hose, and the difference is remarkable. They are as good as they are going to get, and I can trust them now, where I couldn't before.

two more things to add
1) dont forget to lube the pivot bolt for the caliper
2) these bikes are not just like some modern stuff where you throw pads in and go, you have to adjust them, there are tircks to it, check the web , clymers, etc, and make sure your pads are making good equal contact

37071 said:

Does releasing the brake lever not suck the pucks off the rotor - as long as there is no air?

Click to expand...

Correct -- releasing the brake lever does not pull the pads off the rotor.
The brake lever has about 1/2 the resistance it should have. Front wheel does not spin freely.
I gave the caliper a gentle whack with a rubber mallet, and immediately the front wheel was able to spin freely without resistance. Pulled the brake lever in, saw the pads contract and stopped the spinning wheel. The brake lever had little resistance and pulled all the way in. Let the brake lever out again. Pads did not retract from rotor. Front wheel does not spin freely. Pumping brake lever now -- it has returned to about 1/2 resistance it should "normally" have.
Will be disassembling caliper shortly.

nhlkats said:

Correct -- releasing the brake lever does not pull the pads off the rotor.
The brake lever has about 1/2 the resistance it should have. Front wheel does not spin freely.
I gave the caliper a gentle whack with a rubber mallet, and immediately the front wheel was able to spin freely without resistance. Pulled the brake lever in, saw the pads contract and stopped the spinning wheel. The brake lever had little resistance and pulled all the way in. Let the brake lever out again. Pads did not retract from rotor. Front wheel does not spin freely. Pumping brake lever now -- it has returned to about 1/2 resistance it should "normally" have.
Will be disassembling caliper shortly.

Click to expand...

Did you use synthetic or natural fluid? I learned the hard way, my /7 and synthetic brake fluid do not mix. If you totally purged the brake line, it's going to take awhile to get the air out. My method is to pull the break lever in while the valve is open (on the caliper) then close the valve and let the leaver out...keep doing this until you get adequate pressure on the lever (does not feel spongy). I let the bike rest for 24 hours and do it again.

Crap...

Please advise, how the hell do you get the "primary and secondary seals/cups" onto the master brake cylinder piston without hurting it or yourself? It ripped and flew across the garage on unsuccessful mount attempt #4. Hopefully I can order them separately.....

Yes, I was using synthetic brake fluid.

Thanks.

The term "synthetic brake fluid" has no meaning. I have seen fluids advertised as such, however. They are the typical DOT 3 or 4 fluids, both of which are glycol based fluids. DOT 5 is silicone fluid, which will not mix with DOT 3/4 and should not be used. To help get the pumping cup on, try using some DOT 3 or 4 brake fluid as an assembly lube.

Seals are NOT sold separately. To replace seal, one must order new rebuild kit for $68 + S&H.

I will inspect and reuse the old seal on the old piston, swap in the new parts carefully if I can.

nhlkats said:

Crap...

Please advise, how the hell do you get the "primary and secondary seals/cups" onto the master brake cylinder piston without hurting it or yourself? It ripped and flew across the garage on unsuccessful mount attempt #4. Hopefully I can order them separately.....

Yes, I was using synthetic brake fluid.

Thanks.

Click to expand...

dental floss. make a floss loop/seal/floss loop daisy chain

did you try huckys for the seals?

benway said:

dental floss. make a floss loop/seal/floss loop daisy chain

did you try huckys for the seals?

Click to expand...

dental floss was try #2, after fingers. floss broke. both loops.

Will look into this Huckys that you speak of.

Question about the brake light switch at the end of the master brake cylinder: Is there supposed to be some kind of oring, washer, or other seal between m.b.c. and the switch? Or is it sealed just by the threads of the switch? How does that switch work? What provides the contact to set the switch off (piston does not appear to reach that far in the cylinder b/c of the spring...)?

Put it all together, the master brake cylinder, front caliper, and started the brake bleeding process.
But no brake fluid came out of the bleeder valve.
Traced it back up the line. No brake fluid coming out of the master cylinder housing where the brake line attaches.
Only bubbles of air come up out of the reservoir along with small spurts of brake fluid when the piston is pushed in.
I can feel a small push of air and then suction from the hole where the brake line attaches to the master brake cylinder housing when I pull and release the brake level (thus moving the piston in and out).

I have yet to open up the master brake cylinder again, but before I do, I was wondering if there are any sure things I should check based on the symptoms I've described above and this being my first master brake cylinder rebuild attempt. Seems like piston is not pushing fluid out of the housing?