Linked brakes?

[john-g]

I know you’ve already had a plethora of affirmative responses, but one side note worth mentioning………I’m a fairly unique rider(or not?!) and never use the rear brake on any modern Brembo twin disc setup cause feel it’s totally unnecessary. I do on my Harley though cause it’s like a car brake and you actually need it to stop!! (but that’s a subject for another day/ forum)
Anyway my point is, I discovered a weird gremlin in the GS linked system. If you never actually press the rear pedal (like me) air builds up in the rear brake line over time and it stops working properly. I know this only cause my bike failed it’s MOT last year for an ineffective rear brake. My bike was 4 years old at time. Bleeding easily fixed problem and I’ve started routinely pressing pedal now.

Me too, I rarely use the rear brake pedal and the rear Brake on my 2 year old GS needs to be bled between every Service.

 

[TIJ650]

Me too, I rarely use the rear brake pedal and the rear Brake on my 2 year old GS needs to be bled between every Service.

The devil is always in the detail! Lol

 

[TIJ650]

Me too, I rarely use the rear brake pedal and the rear Brake on my 2 year old GS needs to be bled between every Service.

………….Guess it begs the question why even bother linking if you have to press rear brake anyway


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[john-g]

………….Guess it begs the question why even bother linking if you have to press rear brake anyway


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Because using the foot brake pedal only applies the rear wheel brake and is not linked to the front brake. If you want both brakes to be applied then pull the Handlebar lever.

 

[TIJ650]

Because using the foot brake pedal only applies the rear wheel brake and is not linked to the front brake. If you want both brakes to be applied then pull the Handlebar lever.

Maybe I should draw a diagram?!


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[john-g]

Maybe I should draw a diagram?!


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Or you could read page 158 of the R1250GS User Manual

 

[Littledumptruck]

Honda Combined Brake System (CBS). I think it's caused even more terror and grey hair over the years than high rear pad wear on liquid-cooled GSs...
When I was in my twenties, I took a Blackbird out for a fast run to Magaliesberg and Hartebeespoort Dam. The bike was fast and very comfy, but I was expecting 'normal' brake action and I managed to scare myself rigid two or three times.

It was what's called a fully-integral system (the operating principle is the same as on the R1200RT and a lot of Moto Guzzis: the front lever and rear lever both operate the front and rear calipers).
But the Honda way of doing it was fatally flawed on a number of levels: in addition to the front brake operating the rear as Dave describes, when the rear master cylinder was actuated, it fed pressure to a separate slave cylinder which in turn operated the left front caliper. So it ended up being a nightmare of complicated plumbing and extra weight.

But it got worse, because the way CBS operates means you can't use the rear brake to 'trim' the bike's attitude in corners. If you do, it applies the front brake as well, forcing the bike up and wide into the turn. If you try to counter that by leaning in harder and applying more power, you risk wheelspin and highsiding.
Also, because you can't vary the ratio by which you apply front and/or rear brakes, you're stuck with the braking ratio given you by Honda, whether you're on a shopping run or brain-out.

The crowning insult was that Hondas of this era didn't have ABS as a safeguard. Which you needed, because the front and/or rear wheels could lock when you least expected them to.

If I ever owned a Blackbird or vintage VFR800, it wouldn't be going on the road until I re-plumbed it so that front lever = front brake and rear lever = rear brake.

Thats not quite true. Operating the rear brake feeds pressure via a ‘delay valve’ in line. This holds back that pressure for 4-5 seconds before allowing it through to the small central pistons in both front calipers. As the front left caliper is applied the secondry master cylinder on the fork leg applies additional rear breaking. Tbh, if you can’t scrub off enough speed in 4-5 seconds your probably pretty fecked.
I’ve owned three Blackbirds and two Pan Europeans with this system and have never found any need to rip it out. I did however have a mate that bought a new Blackbird back then, he moaned about how the rear brake would upset the front end mid bend, but after a visit back to the dealer and a new delay valve fitted under warranty the issue was sorted.

 

[FoxCorp]

I struggle to get used to this - after many years on a normal system I'm so used to using the real brake for fine tuning & stabilising that when I do pull the front brake it applies more rear and according to the connected app my ABS is kicking in all the time when I do this.

Maybe I should stop using the rear and just relying on the front lever but I can see this causing some real excitement when I get back on my other bike.

 

[GrinningGSer]

I experience MUCH more braking power if I use the front brake PLUS my rear brake , on my linked GSA . I’ve asked about it before and never got an explanation.
Most of the time , I’m a ‘two finger breaker’.

That said , according to my NavV, my rear brake is usually applied three times more than my front brake , on any ride and that’s without me touching the rear brake ; so must be a bike preference thing .

 

[SBD]

I’ve got an old Guzzi. The linked brakes are applied by the pedal (one front disc and some percentage on the rear) so my habit is the apply all the brakes by foot, and add, if needed, by hand (which is the other front disc on the Guzzi). The BMW is arse about face for me.

 

[TIJ650]

Still doesn’t explain why air builds up in rear line if rear pedal not activated (over time)


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[SBD]

I can see no reason for air to get into the system, whether it’s used or not.

 

[DJ123]

Still doesn’t explain why air builds up in rear line if rear pedal not activated (over time)


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I'd not say in an air build up. Probably a change in the pressure coming from the pedal to the calliper rather than from the ABS pump to the calliper.

 

[TIJ650]

I can see no reason for air to get into the system, whether it’s used or not.

I didn’t think so either. Suspected air getting in at the reservoir or master cylinder. Nipple was tight enough too.
Just that the MOT tester (Motorrad) had come across this few times before and said one common denominator was all the culprits hadn’t been pressing rear pedal. He was equally puzzled to reason

Problem hasn’t reoccurred since bleeding 6 months ago.


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[GrinningGSer]

Still doesn’t explain why air builds up in rear line if rear pedal not activated (over time)


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Well , I’ve read on a helpful Ducati YouTube (because they suffer notorious rear brake failures ), that Dot 4 fluid is NOT all the same . Better stuff has higher boiling points but costs ££

Ordinary dot 4 boils at 155deg C
Motul RBF 600 boils at 205degC

Just maybe it’s a feature of heat and closeness to a heat source ? Dunno
Maybe worth trying better brake fluid ?

 

[TIJ650]

Well , I’ve read on a helpful Ducati YouTube (because they suffer notorious rear brake failures ), that Dot 4 fluid is NOT all the same . Better stuff has higher boiling points but costs ££

Ordinary dot 4 boils at 155deg C
Motul RBF 600 boils at 205degC

Just maybe it’s a feature of heat and closeness to a heat source ? Dunno
Maybe worth trying better brake fluid ?

To be honest, the work around is straightforward (bleeding as service item), but was sure some Mensa guru on here would’ve had answer?!


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[SBD]

I’m still struggling with the idea that somehow air gets into the brake system. There’s no “suck”, no pressure below atmospheric, that would cause that to occur. It seems vanishingly unlikely that there’s a compressed air source within the ABS unit which could somehow force air in,and if it did, it would cause brake drag. A leak would certainly be more likely to let fluid out than to let air in, although that could be hidden, as it could be within the master cylinder or ABS unit.

It also seems vanishingly unlikely that such a failure would get through system and vehicle level testing.

I think the MOT tester / Motorrad tech was either deluded or telling porkies.

you might reasonably ask - why did bleeding fix it? I can’t answer that, but I’d suggest that it didn’t, although something else done at the same time may have had an effect. I’d like to see a video showing bubbles coming out, that would convince me there was air in the system, though not how it got there.

 

[TIJ650]

I’m still struggling with the idea that somehow air gets into the brake system. There’s no “suck”, no pressure below atmospheric, that would cause that to occur. It seems vanishingly unlikely that there’s a compressed air source within the ABS unit which could somehow force air in,and if it did, it would cause brake drag. A leak would certainly be more likely to let fluid out than to let air in, although that could be hidden, as it could be within the master cylinder or ABS unit.

It also seems vanishingly unlikely that such a failure would get through system and vehicle level testing.

I think the MOT tester / Motorrad tech was either deluded or telling porkies.

you might reasonably ask - why did bleeding fix it? I can’t answer that, but I’d suggest that it didn’t, although something else done at the same time may have had an effect. I’d like to see a video showing bubbles coming out, that would convince me there was air in the system, though not how it got there.

Certainly beyond my technical expertise. It all started with a simple observation. 18 months ago my bike passed MOT, but was given ‘advisory’ on rear brake……….one year later - it failed. Refuse to let those air gun/ loctite thieves near my bike - so took away and bled rear brake. Returned and passed MOT. Just chatting with mechanic (who seemed fairly impartial and rode jap bike himself anyway) and he mentioned he’d come across this few times and asked if I press rear brake?! I said I didn’t cause effing brakes linked anyway
Probably inaccurate diagnosis, but got me thinking.


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