1150 GS caliper piston mystery

[motoplus40]

2001 1150 GS, 37,400 mi.
I was following the rear brake pad replacement manual from the hall of wisdom and everything was going well. I was taking advantage of a rear tire renewal and I only wanted to disassemble and lubricate the rear caliper assembly and inspect the pads.
I separated the carrier and caliper but when I went to push back both pistons with a block of wood they wouldn't move. I didn't have a small c-clamp, tried a larger one which really could not get a centered purchase on one of the pistons, and then figured, no problem, when I reinstall I will use the hydraulic brake system pressure to get them moving.
But after I remounted the caliper, the pistons would not budge.
When I pumped the pedal the (original rubber) brake line bulged just above the banjo crimp, which looked a little ominous but I was getting pressure.
Brake worked fine before I disassembled. I didn't touch the foot brake while I did the work. The pistons are equally showing about 5/16" out of the caliper.
Wondering what I try next: more c-clamp pressure, disassemble and rebuild caliper, and how the hell did I screw this up?

[TUCKERS]

Don't know.

But be careful with C clamps on the piston face, they are rather fragile and you could punch a hole in there easily.

(Don't ask me how I know)

[JurgenB]

The c-clamp action has possibly tilted the piston and jammed it. I suggest that you install a suitable grease nipple instead of the bleed screw and press the piston out with a grease pump. After that, carefully clean all parts and pieces, replace gaskets and put the caliper back together. If the piston was jammed, it and/or the caliper bore may be damaged beyond repair.

[AntonLargiader]

You have a bad brake hose! Not uncommon. It may be blocking the fluid from returning to the reservoir, but in any case it's bad. You saw it with your own eyes.

[motoplus40]

Ok my brain has been released from inaction:
No more attempts to encourage pistons to go back, and now accepting the fact I have to take this apart.
It may be that the pistons were canted when I tried to push them in. but they resisted even the early attempt to push by hand so I had the idea that they were stuck before..
Yes, I did see the bubble in the brake line just before the crimp which suggested a bad line. Maybe that has something to do with the immovable pistons... (I will also finally stop resisting that I have to change to steel braided lines this year.)
So perhaps the mystery will be solved when I drain the master cylinder and the line to the caliper. If not, it is on to the "grease pressure treatment" to release pistons and then rebuild (I don't even want to think about if I ruined the caliper or pistons, I don't see any replacements for the pistons, that would mean $450 for a caliper, then I would HAVE to win a new bike at the national rally in July.)
Guess I gotta get back to the garage.

[FatChance]

Originally Posted by motoplus40

So perhaps the mystery will be solved when I drain the master cylinder and the line to the caliper. If not, it is on to the "grease pressure treatment" to release pistons and then rebuild (I don't even want to think about if I ruined the caliper or pistons, I don't see any replacements for the pistons, that would mean $450 for a caliper, then I would HAVE to win a new bike at the national rally in July.)
Guess I gotta get back to the garage.

First, disconnect the brake line at the caliper and try moving the pistons, watching to catch any brake fluid that squirts out of the open hole. Personally, I would never introduce anything like grease into a brake system part no matter how well you clean it. It would be better to use compressed air if you want to try to force them out.

Or, just go to the worst case and replace it. I recently had to replace a rear caliper on my 1150GS and got more than a half dozen offers to sell used ones in the flea market from $65 to about $120. Beemerboneyard has used ones for $115. I got a very good, complete used rear caliper for $100/shipped from someone I know in this forum. No need to buy a new one for $425!

[I GS 1]

Originally Posted by FatChance

Personally, I would never introduce anything like grease into a brake system part no matter how well you clean it. It would be better to use compressed air if you want to try to force them out.

Or, just go to the worst case and replace it. I recently had to replace a rear caliper on my 1150GS and got more than a half dozen offers to sell used ones in the flea market from $65 to about $120. Beemerboneyard has used ones for $115. I got a very good, complete used rear caliper for $100/shipped from someone I know in this forum. No need to buy a new one for $425!

I agree, I wouldn't go anywhere near it with grease. If you did you would end up replacing the whole system

[rebelpacket]

Stop looking to the caliper for the problem.

Anton's assesment is correct. A bulge in the brake line indicates its failed. Since yours is a 2001, if you haven't replaced the lines yet, its most definitely the problem.

Replace the lines with the galfer kit. Do the front ones while your at it if they haven't been done already.

Originally Posted by AntonLargiader

You have a bad brake hose! Not uncommon. It may be blocking the fluid from returning to the reservoir, but in any case it's bad. You saw it with your own eyes.

[def]

Originally Posted by FatChance

Personally, I would never introduce anything like grease into a brake system part no matter how well you clean it. It would be better to use compressed air if you want to try to force them out.

Grease won't hurt anything and is a good idea. You can merely flush out the grease after things are working normally. But first, I would try compressed air...keep your fingers away from the pistons while trying compressed anything.......

Anton is likely correct (He usually is) you've got debris in your brake system from the OE rubber lines.

[TUCKERS]

It helps to fill out your profile.

Where are you?

I can get stainless lines cooked to order at a local aircraft supply.

[motoplus40]

I am in NYC, specifically in Queens, 5th ManCave on the right.

[Beezer]

happened on my Y2K as well.... the rear line ballooned up. I got a replacement made at the local commercial hydraulic shop for $30. BTW, my front lines went first... they came apart inside and and blocked the fluid from returning so the brakes would stay on. They also barfed rubber hunks into the ($2200) ABS module.

[motoplus40]

It was the brake line as Anton et al predicted. Separated the caliper from the banjo, pistons went in, pressed on foot brake the bubble started in the brake line, little more pedal pressure, something popped and brake fluid flowed, bubble deflated. I probably added to the problem by twisting the brake line a little too much when disassembling.

So now I am into new stainless lines front and rear. But how far do I go with this renovation? Does Beezer's-barfing-front- brake-lines experience suggest I have to go looking for brake line bits throughout, i.e. rebuild the caliper anyway, and somehow clean out the ABS unit? Beezer, did you actually have to buy a new ABS unit because of the barfing?

[BobbySands]

I did this last year after my brake line did what yours did. It bulges, but really it comes apart inside forming a sort of check valve. Fluid gets past the flap, but cannot return back to the master cylinder. Hence: pistons stuck.

You have an '01. No servos. The BMW uses hard-piped brake lines to the ABS unit, so it's really very simple. You have one brake line from the front master cylinder to a hard pipe, which feeds the ABS unit. You have another line from the hard pipe coming from the ABS unit to the cross over, and then the crossover, which is a hard line, to each caliper. The two hard lines are sort of in the vicinity of the front shock. Very easy.

In the rear, you have a hard pipe to the ABS unit from the master cylinder, so no stainless line needed. You will have a short line to the rear caliper from a hard pipe which comes off the ABS. The hard line comes from the ABS almost to the right rear passenge footpeg. Connect to there and then to the rear caliper. Also very simple.

That's the kit you can order from Spiegler. It came in about 4 days and had everything I needed aside from brake fluid. I think the change took me about 3 hours at an extremely liesurely pace and about 7 pints.

From master cylinder to ABS line:


Sort of just above the front shock:



Front crossover


rear brake


I made this one large, if you look carefully you can see both front connections and the rear:


Bike:

[I GS 1]

Originally Posted by def

Grease won't hurt anything and is a good idea. You can merely flush out the grease after things are working normally. But first, I would try compressed air...keep your fingers away from the pistons while trying compressed anything.......

Anton is likely correct (He usually is) you've got debris in your brake system from the OE rubber lines.

Caution. I would be very careful following this advice the flushing fluid (not specified) and the grease could harm seals/boots etc. and I am not sure how you old now if you got it all out. IMHO You would be best sticking to air