Sanity check, rear brake edition

But my brakes do work, nothing locks up or spins freely when it shouldn't. I might try to block off the rear abs portion and run a line straight from the master to the caliper.
No they dont work

you have a pad not backing off from the disc ... its not right

going forward

if going direct master to caliper restores normal operation

that would suggest the ABS unit is having an influence

if not you need to eliminate

master cylinder
Brake line
Caliper

You need to think about how the circuit works

Master cylinder - ABS unit - caliper not master cylinder - caliper & add ABS when needed

The ABS unit is monitoring the rotation speeds of the wheels constantly A) for ABS & B) to provide your road speed in the Kombi

if the ABS unit detects lock up when braking it modulates the brake pressure to give cadence braking on/off/on/off/on/off very rapidly to slow you and give steering control

So as long as you have your brake applied enough to trigger the ABS module - its doing the braking not you

once you come off the brakes, and wheel rotation is restored the ABS stops modulation

so if you have an internal fault in the ABS unit the pressure might not be released enough, and the symptom of a sticky caliper

Thats similar to what had on the front, we initially thought sticky caliper

6 months later - it was the ABS unit gone south

As i said

Eliminate the other parts

Rear master cylinder

rear brake line

rear caliper

if one of those clears the fault :) job done, i was wrong

if not, it can only be the ABS unit

One thing you havn't mentioned ,or if you did i missed it

What year of bike hexhead or twincam
mileage
service history (brake fluid changes)
 
No they dont work

you have a pad not backing off from the disc ... its not right

going forward

if going direct master to caliper restores normal operation

that would suggest the ABS unit is having an influence

if not you need to eliminate

master cylinder
Brake line
Caliper

You need to think about how the circuit works

Master cylinder - ABS unit - caliper not master cylinder - caliper & add ABS when needed

The ABS unit is monitoring the rotation speeds of the wheels constantly A) for ABS & B) to provide your road speed in the Kombi

if the ABS unit detects lock up when braking it modulates the brake pressure to give cadence braking on/off/on/off/on/off very rapidly to slow you and give steering control

So as long as you have your brake applied enough to trigger the ABS module - its doing the braking not you

once you come off the brakes, and wheel rotation is restored the ABS stops modulation

so if you have an internal fault in the ABS unit the pressure might not be released enough, and the symptom of a sticky caliper

Thats similar to what had on the front, we initially thought sticky caliper

6 months later - it was the ABS unit gone south

As i said

Eliminate the other parts

Rear master cylinder

rear brake line

rear caliper

if one of those clears the fault :) job done, i was wrong

if not, it can only be the ABS unit

One thing you havn't mentioned ,or if you did i missed it

What year of bike hexhead or twincam
mileage
service history (brake fluid changes)
Thanks for the comprehensive write up!

It's a 2010 model, twincam. About 45000 miles. Brake fluid changed as soon as I got it last year, fluid that came out looked fresh. Changed the fluid again earlier this week when I rebuilt the caliper. The slide pin rubbers where not allowing sliding as they should. I cleaned them with the wrong cleaning agent earlier this year when changing pads. So last week I replaced all the seal, rubbers parts, stainless steel pad springs and pad pin. Besides that the disc was replaced with a new one and new OEM pads where installed. I'm still thinking the pads and disc need to be bedded more.

The pressures I'm reading in motoscan are alarming, also the pressure drops as soon as I let of the pedal.

The pads do come of the disc, original question was do they come off far enough? And it's hard to make a precise assessment by friction in the rear wheel or by noise. As soon as I let go of the pedal the rear rotates again. Last nights ride of about 20 miles made the brake caliper like warm. Disc slightly warmer than that. Mind you, I did all my braking on the rear.

If it was really binding wouldn't it have been hotter/scolding hot? How warm should the rear be after a ride?

Just for fun I'm going to a local shop this afternoon that has a few gs'es of this vintage, just to feel listen and compare
 
So ive just had a new rear disc on mine literally yesterday

Just been down to the garage - rotating the rear wheel you can hear slight pad/ disc contact - the pad is resting on the disc no pressure applied just resting

it sounds the same as if you rotate a wheel on a car - a light contact noise

you dont want clearance as in a measureable gap, as this has to be taken up before the pad bites

yours sound as if there is still residual pressure on the pad, so its applying the brake ever so slightly

Have you confirmed the caliper is sliding on the guides?
 
Just a thought,

you said your replaced the sliders, did you confirm the caliper was sliding on the slider?


This is going to sound really stupid or contraversial depending on how approacheable you are

You have assembeld the pads the right way round and not on metal to metal ?

if you can, pop the pads out and get a photo up & the disc,

might give a few more pointers
 
if you can, pop the pads out and get a photo up & the disc,
i was going to suggest that (photo) earlier, but the OP isn’t a member - I didn’t think “registered users” had privileges to upload photos?
 
Just a thought,

you said your replaced the sliders, did you confirm the caliper was sliding on the slider?


This is going to sound really stupid or contraversial depending on how approacheable you are

You have assembeld the pads the right way round and not on metal to metal ?

if you can, pop the pads out and get a photo up & the disc,

might give a few more pointers
I'm really approachable, all pointers are welcome!

In the rebuilt caliper the sliders slide freely, when pushed fully in they spread ng back after that. Before rebuilding that didn't happen.

Pads are right way around.

Just came back from the shop I was talking about earlier. I am no longer welcome there because allegedly I have a "wheel rotation fetish"... (Joking obviously). But there were 4 GS's, 2 RT's, 1 R and one R nine T. Touched them all (appropriately), some rotate smoother than others, some make more noise from pads than others.

Their mechanic (who only works in BMW's and Ducati's) took at look at mine and said nothing was out of the ordinary. Brake functions, pads retract far enough after releasing pedal.
 
Ok

I see some questions,

what pads are they? OE or aftermarket,

They look aftermarket, in which case are they sport pads ?

Hint it's a GS - tractor and it's the rears. They don't need sport pads. Std is fine

Your pad doesn't appear to be sitting against the disk one one side. May be the pic but it looks odd

Your pad slider pin. Looks odd

They don't have grooves in them if I recall

If I can get mine back in the shade. I'll whack some brake cleaner over it and have a look

Have you tried backing the rear pistons all the way in and then pumping the pedal to make sure the pistons are free?

Then push them back fit pads and pump the pedal to seat them
 
Just seen your comments re taking to shop

Give them 100 miles of increased braking and report back
 
Ok

I see some questions,

what pads are they? OE or aftermarket,

They look aftermarket, in which case are they sport pads ?

Hint it's a GS - tractor and it's the rears. They don't need sport pads. Std is fine

Your pad doesn't appear to be sitting against the disk one one side. May be the pic but it looks odd

Your pad slider pin. Looks odd

They don't have grooves in them if I recall

If I can get mine back in the shade. I'll whack some brake cleaner over it and have a look

Have you tried backing the rear pistons all the way in and then pumping the pedal to make sure the pistons are free?

Then push them back fit pads and pump the pedal to seat them
All parts where purchased directly from the biggest BMW dealer in the Netherlands (where I live). Everything came in sealed BMW bags. So the pin, the stainless pad springs, the pads itself and the rubber parts, ordered by parts numbers I got from their site as well (cross checked that with another partsnumber site from a German dealer).

The shims on the back my these pads surprised me to, but underneath the shims it says Brembo.

Grooves in the pin are normal, it's even described in the owners manual that came with my bike. It's a wear indicator.

Pistons move freely no noise or rubbing when pushed all the way in. I agree the picture looks odd, but it seems to be reflection on the disc giving the illusion of an air gap. Pads touch completely as seen on their picture.
 
Just a quick question, most likely checked by yourself and mechanic.

Did you set the required gap normally on the brake master cylinder and plunger rod, when adjusting the rear brake pedal?
Sometimes people over adjust to make their pedal higher, but this adds pressure to the brakes and can over heat the rear brakes, with bad effects.

The gap adjustment on a rear brake master cylinder can significantly affect overall braking performance by influencing the hydraulic pressure applied to the rear brakes. Proper adjustment ensures that the rear brakes engage effectively without causing premature lockup, which can lead to instability during braking.
A Free play gap of around 15 mm is required.
 
I can see what your problem is.
Everything in there is far too clean. Mine is filthy with built up crud everywhere and works fine.
Wait until it gets dirtier then it'll be fine. 👍🏼🤷🏻‍♂️🤣
 
The rear pads on these BMW rear calipers should be different thickness.
For instance Ferodo 2039 rear pads and Lucas/TRW and certain EBC have one pad thicker than the other ..
One pad wears out faster than the other due to constantly touching the disc. Not enough to get the disc hot, just warm to the touch.
One is thicker than the other from new so they both come to the end of their life at roughly the same time.

My work here is done.
 
Last edited:
The rear pads on these BMW rear calipers should be different thickness.
For instance Ferodo 2039 rear pads ..
One pad wears out faster than the other due to constantly touching the disc. Not enough to get the disc hot, just warm to the touch.
One is thicker than the other from new so they both come to the end of their life at roughly the same time.

My work here is done.
Warm to the touch from just riding without specifically using the rear brake? If so, that is what happens.
 
Just a quick question, most likely checked by yourself and mechanic.

Did you set the required gap normally on the brake master cylinder and plunger rod, when adjusting the rear brake pedal?
Sometimes people over adjust to make their pedal higher, but this adds pressure to the brakes and can over heat the rear brakes, with bad effects.

The gap adjustment on a rear brake master cylinder can significantly affect overall braking performance by influencing the hydraulic pressure applied to the rear brakes. Proper adjustment ensures that the rear brakes engage effectively without causing premature lockup, which can lead to instability during braking.
A Free play gap of around 15 mm is required.
The mechanic did indeed check this and verified there is enough free play before the plunger is actuated.
 
Warm to the touch from just riding without specifically using the rear brake? If so, that is what happens.

Normal .. one of the rear pads touches the rear disc, and the rear caliper doesn’t get a direct blast of cooling air as you ride.
 


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