Rear brake binding

[andeebee]

So following my clutch rebuild I added a rear brake rebuild onto the list as it was looking knackered and the disc was feeling like it was warped when rotating the wheel.
So new Brembo disc, brembo caliper repair kit and new stainless bleed screw was ordered and fitted.
As noted in my clutch rebuild thread, it failed the MOT due to a binding rear brake and I agree it is binding.

I have pushed the pistons back out again to check I hadnt used too much (brembo) assembly grease and reassembled. No better.
The slider pins are clean and well lubed and were in pretty good condition so needed no rubbing down with wet and dry etc.

When rotating the wheel the far pad (not piston side can be seen to be grabbing the disc as it moves in its location.
The piston side does not appear to be binding.

How far back should the pistons be receding on release of the brake? Mine are easing off next to nothing.
I certainly cant spin the wheel round a full rotation due to the drag.
Given that I have rebuilt the calipers could the problem be the rear master cylinder?

 

[Edward]

Sometimes degeneration of the rubber brake hose can cause brake binding.

 

[hogfarmer]

I had similar prob on f650, it turned out to be master cylinder not allowing pressure to release. Second hand one from motoworks fixed it.

 

[andeebee]

Thanks for replies, Im thinking m/cyl also.
As my bike is a 2008 GSA I think the later GS (<2010) m/cyls will fit. I would like to find one with as few miles as possible.
Saying that I havent tried motoworks for a 2nd hand one yet...

 

[Magwych]

Have you checked the caliper slider bracket thing for:
1 Front edge of the brake pads correctly located.
2 Wear on the ledge where the bottom edge of the fronts of the pads sit?
If the pads are not cirrectly located; or, that ledge ( and shim protecting it) have worn through to create a slot; then, they will bind.
I replaced that carrier on mine because it had worn so much that the inboard pad jammed down into a slot. When I put it back together it was just as bad; because I had managed to get the outboard pad into the slot the disc passes through. Once I realised what was happening it took a minute to fix. I had started to talk myself into a master cylinder or ABS pump issue.

Sent from my SM-G388F using Tapatalk

 

[andeebee]

Sounds feasible, thanks for suggestion Magwych. Because that side of the caliper is out of view it would be easy for a pad to become lodged in the wrong place, especially if corrosion/wear had occurred there. Will take a look.

 

[andeebee]

Checked for pad alignment/ carrier wear issue as above and all looks OK with pads moving in their shims freely and no apparent wear to carrier.
Replacement used m/cyl arrived today and following a hefty bleeding session there is no improvement to the rear brake.
In fact my heart sank as I noticed when the m/cyl was disconnected, during the m/cyl swap, that the brake was still binding.

So with zero hydraulic pressure the caliper is binding by itself suggesting pin alignment? If I knock the caliper lightly with a rubber mallet it moves on its pins, compresses the piston and the rear wheel is free.

I took it out for a quick spin to check the rest of the brake system and get no (dash) fault lights and brakes work well otherwise, although still unable to lock rear wheel with rear brake alone. It slows the bike down eventually but not like other bikes.
Im going to book it in with a local shop, to get the faults read and someone else to look at it as Im reaching the end of my abilities now.

I guess I could get a replacement caliper but the current one isnt looking bad so Im reluctant (the m/cyl was probably OK but is now eliminated at least).
Have also just purchased new front discs and replacement (used) front calipers to replace the front end in the near future due to the wear I noticed during the MOT prep time.

Amazingly the front brakes passed the MOT despite one of the pistons looking like a 50p.

So frustrating, just want to get out on the damn thing.

 

[Bendy toy]

Have you removed the pistons and fitted new seals? The seal kit is £20.
BMW seem to have gone for a rear disc size that looks good. But go for a ride and that back end will always be hot. The linked hydraulics basically overwork the back brake.
If you suspect the back brake is binding, ride the bike for a mile (or three)!without touching front or back brakes. Stop with as little brake as you possibly can. Check the back disc it should be hardly warm. Now return but brake normally. The fronts will be almost cold the back will be hot.
Go figure as they say over the pond.

Sent somehow.

 

[andeebee]

Bendy, yes new Brembo seal kit fitted. Pistons both good, only a couple of marks half way down. Assembled using the supplied Brembo grease.
I have had these apart a couple of times and the caliper is in very good condition.
On pulling the main seals out again yesterday I set them on the bench with the pistons inside and noted the seals are oversize to the pistons by a few thou.
I suspected the wrong seals for a moment but the James Sherlock site suggests one size fits all R1200 Rears.
Having reassembled and had the same issue I went on t'web and ordered another used caliper.
So far I have replaced rear m/cyl, rear caliper, rear seals, rear disc, front calipers and front discs. Only the ABS module and lines to go and its a complete set!

 

[Santa-2512]

Bendy, yes new Brembo seal kit fitted. Pistons both good, only a couple of marks half way down. Assembled using the supplied Brembo grease.
I have had these apart a couple of times and the caliper is in very good condition.
On pulling the main seals out again yesterday I set them on the bench with the pistons inside and noted the seals are oversize to the pistons by a few thou.
I suspected the wrong seals for a moment but the James Sherlock site suggests one size fits all R1200 Rears.
Having reassembled and had the same issue I went on t'web and ordered another used caliper.
So far I have replaced rear m/cyl, rear caliper, rear seals, rear disc, front calipers and front discs. Only the ABS module and lines to go and its a complete set!

I'd do the lines next, far cheaper than an ABS module


mart

 

[MWH]

Is the rear hub straight? On a previous GS as the rear wheel turned it would push against the pads, bmw changed the wheel/disc and when I collected it it was still doing it. Turned out the rear hub wasn't mounted straight.

 

[andeebee]

Sorted this problem with a low mile used replacement caliper from Sherlocks.
Had it on and bled in 20 minutes and brake works a treat and wheel spins for the first time since I have had the bike.
Easier to push around too!
Feck knows whats up with the old one.
MOT re-test next. Thanks for all replies and suggestions.

 

[Pukmeister]

I'd guess at slight wear in the bore of the old caliper slider, causing flexing and jamming when operated.