Jump to content
IGNORED

R1100RT Rear Caliper Pistons Wont Retract


RicoRider

Recommended Posts

Rear caliper pistons wont recess enough to fit new pads and rotor.

 

I'm replacing the rear disk and pads on a 97 R1100RT at 106K miles (dont use'em much). I've replaced the front rotors once and the pads a few times with no problemo but can't get the rear pistons to fully retract enough to fit the new pads and disk. 

 

Each piston will retract fine but pressed together they stop at about .5". I've partly drained and opened the rear reservoir and even opened the rear ABS circuit to try reliving enough pressure but nothing seems to work.

 

Hoping the problem is the nut behind the wrench and I'm overlooking something that'll turn out to be obvious. Many thanks...

Link to comment

If you still have the original rubber brake hose, it may have partially collapsed internally and isn't allowing fluid to go back towards the reservoir.  The rubber hoses should have been replaced years ago.  Clean off the pistons with brake-clean and open the bleeder at the caliper.

  • Plus 1 1
Link to comment
12 hours ago, RicoRider said:

Rear caliper pistons wont recess enough to fit new pads and rotor.

 

I'm replacing the rear disk and pads on a 97 R1100RT at 106K miles (dont use'em much). I've replaced the front rotors once and the pads a few times with no problemo but can't get the rear pistons to fully retract enough to fit the new pads and disk. 

 

Each piston will retract fine but pressed together they stop at about .5". I've partly drained and opened the rear reservoir and even opened the rear ABS circuit to try reliving enough pressure but nothing seems to work.

 

Hoping the problem is the nut behind the wrench and I'm overlooking something that'll turn out to be obvious. Many thanks...

Morning RicoRider

 

It sounds like something is blocking the rear fluid from returning into the rear master cylinder reservoir.  

 

The first thing to check is that the rear master cylinder piston is FULLY returning to it's out-stop. If that rear master cylinder piston is not fully returning (out) then the piston will block the small take-up port hole inside the master cylinder therefore blocking return of fluid from the caliper back to the reservoir. 


It is possible that someone has incorrectly adjusted the rear pedal stop bolt therefore preventing the master cylinder piston from fully returning. 

 

There are other places that the fluid return could be blocked (like inside the ABS unit, or that hose that Michaelr11mentioned). 

 

If opening the rear caliper bleeder fitting allows the pistons to both fully retract together then that should allow the new pads to install but that only allows the new pads to install, it doesn't fix the trapped fluid problem so even if you get the new pads to install you will still have a trapped fluid issue that will probably cause rear brake drag, excess rear brake heat,  & quick rear brake pad wear.   

 

 

 

YYTgWvD.jpg

 

  

Link to comment

@Michaelr11 and @dirtrider, THANK YOU for the input. Unfortunately I'm still at top dead center.

 

1. Tried getting pressure relief at all master cylinder positions including disconnected from brake lever...nada.

2. Inspected all lines and fittings...all good.

3. WAS ABLE to fully depress the pistons by opening the bleeder screw (appreciate that doesn't solve it)

4. WAS ABLE to manually bleed the circuit a) at the rear caliper and b) at the ABS unit using the brake lever.

 

Since the system bleeds normally I'm thinking it not the lines. That leaves the master cylinder but that's pumping the bleed just fine. Any other ideas? Otherwise I'm hoping mercury is in retrograde and its a gremlin that'll disappear if I finish the rotor/pads and bleed. That goes against my "hope is not a winning strategy" so am very open to your thoughts. 

Link to comment
34 minutes ago, RicoRider said:

@Michaelr11 and @dirtrider, THANK YOU for the input. Unfortunately I'm still at top dead center.

 

1. Tried getting pressure relief at all master cylinder positions including disconnected from brake lever...nada.

2. Inspected all lines and fittings...all good.

3. WAS ABLE to fully depress the pistons by opening the bleeder screw (appreciate that doesn't solve it)

4. WAS ABLE to manually bleed the circuit a) at the rear caliper and b) at the ABS unit using the brake lever.

 

Since the system bleeds normally I'm thinking it not the lines. That leaves the master cylinder but that's pumping the bleed just fine. Any other ideas? Otherwise I'm hoping mercury is in retrograde and its a gremlin that'll disappear if I finish the rotor/pads and bleed. That goes against my "hope is not a winning strategy" so am very open to your thoughts. 

 

How did you inspect the lines?  Did you remove them and try bowling from both directions?  Are they the original rubber lines or have they been swapped to SS lines?  If rubber they have a liner that will collapse and only allow fluid to only flow one way.  Seen this on many cars as well.  If opening the bleeder let you push the pistons all the way in they I bet you have a collapsed brake line and should replace them all.

 

 

Link to comment
2 hours ago, RicoRider said:

@Michaelr11 and @dirtrider, THANK YOU for the input. Unfortunately I'm still at top dead center.

 

1. Tried getting pressure relief at all master cylinder positions including disconnected from brake lever...nada.

2. Inspected all lines and fittings...all good.

3. WAS ABLE to fully depress the pistons by opening the bleeder screw (appreciate that doesn't solve it)

4. WAS ABLE to manually bleed the circuit a) at the rear caliper and b) at the ABS unit using the brake lever.

 

Since the system bleeds normally I'm thinking it not the lines. That leaves the master cylinder but that's pumping the bleed just fine. Any other ideas? Otherwise I'm hoping mercury is in retrograde and its a gremlin that'll disappear if I finish the rotor/pads and bleed. That goes against my "hope is not a winning strategy" so am very open to your thoughts. 

Evening RicoRider

 

If it bleeds OK using the rear brake pedal then the master cylinder is probably working OK as the piston would have to uncover the port to allow it to pump fluid through it. 

 

So that pretty well leaves a failed hose (when interior liner fails it can make a one way check valve) so it can pump fluid to the caliper but the caliper can't push it back the other way. 

 

Or something caught inside  the caliper that acts like a one-way check valve (can easily happen on a 4 piston caliper but not easily on a 2 piston caliper (both pistons on same side).  

 

Or some sort of problem inside the ABS unit allowing some junk to from a one way check valve.   

 

As mentioned above, probably start with the hose as that is simple & fairly cheap  (should be replaced anyhow on an older BMW motorcycle). 

Link to comment

Rico.. I can’t help you diagnose, DR will figure it out with you. That said, I have a question. When you opened the bleed valve and they retracted, we’re you able to push them back by hand or did you need a clamp and some pressure?

Link to comment

Morning guys...ok, hearing you. Thought the hoses allowing the bleed eliminated the blockage q but now get the inner-lining-failure-one-way-check-valve-effect is real. And yea, wasn't looking forward to adding new hoses to my (already pretty substantial) 100k winter rehab project, but will stop whining now and just get it done. 

 

Thanks again.

Link to comment

Spiegler or galfer. Tome Cutter at the Rubber Chicken Racing Garage is / was a Spiegler dealer and a longtime BMW wrench. Ordering from him is a good way to make sure you get the right stuff.

  • Plus 1 1
Link to comment

Can I get a witness?!

 

The larger piston was much harder to blow out than the other. It has a faint sleeve mark around half the circumference near the bottom of the piston. I'm guessing it raked slightly as it neared its max extension. This mark is only on half the piston

 

1045383706_Slevemark.thumb.jpg.f0ea4f33ba01ea24acfb2b169b794c98.jpg

 Not this half...

339522714_Slevemarkless.thumb.jpg.56b0eb9984ad3be0ee48aed9e8cefa28.jpg

 

Looking for opinions, is this piston good?

 

Rico

Sleves.jpg

Link to comment
14 hours ago, RicoRider said:

Can I get a witness?!

 

The larger piston was much harder to blow out than the other. It has a faint sleeve mark around half the circumference near the bottom of the piston. I'm guessing it raked slightly as it neared its max extension. This mark is only on half the piston

 

 

 Not this half...

 

 

Looking for opinions, is this piston good?

 

Rico

 

Morning Rico

 

It doesn't look that bad, not desired but not unusable either.

 

Put the piston back in the caliper with no seals then see if it strokes smoothly without sticking.  If there are no signs of sticking then personally would use it. 

 

Also look closely  & feel for anything in the piston bore that could wear it like that. 

 

Just make sure the piston doesn't stick at any point in it's travel.

 

BMW doesn't service the rear caliper pistons so you are pretty well stuck with that piston unless you find another caliper. If you find a  used caliper it might be as bad or worse. 

 

 

  • Smile 1
Link to comment
  • 2 weeks later...
Brodiepunker18

I found replacement pistons for the front and rear calipers when I did my brakes last spring….let me look to find my receipts if you’re looking to replace them. 

Link to comment
Brodiepunker18

Euromotoelectrics is where I got the rear set(pistons and seals) because they weren’t available oem. 

Link to comment
On 2/11/2023 at 10:49 PM, RicoRider said:

Sorry, that cylinder shot was cr_p and I thought I deleted it. 

 

image.png.9d2c01123517df7b839d7c00d2cb45cc.png

 

Not a bad shot; hard to get everything in focus on curved surfaces, close-up, and on black surfaces. 

 

It clearly shows both the inner and outer seals.  As I remember both seals are directional - they have an inside and outside direction (as "inside" the cylinder and "outside" the cylinder - the inside face is tapered).  Also, the cylinders and pistons are slightly different diameters in the same caliper half. The outside seal has that tiny groove.  Yours look to be in very good condition.  IF you remove them to clean the caliper just clean and replace the seals one cylinder at a time.  I think Dirtrider pointed out several years back that the stiction between the rubber seals and the piston it what slightly retracts the piston and releases the disc when you release the pedal/lever.  Sometimes a little stiction is good.

 

 

  • Like 1
Link to comment

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...