Brakes again .......

[knutk]

I didn’t leave it over night with a weight hanging off the pedal but when I went in to put the bike on the lift this morning, I tested the pedal and it seemed fine. Huh?!

I left it on the skylift, nose down for an hour or so then using my weight to put pressure on the pedal, I opened the bleed nipple. I got about three very tiny bubbles but that's all.

I’ll leave it on the stand with the weight on the pedal until this afternoon and repeat this gentle bleed.

Hopefully, any further small bubbles will find their way up to either the Master cylinder reservoir or the calliper

For bleeding the brakes, the compressor driven bleeder is essential. A hand pump will not be able to provide the continuous flow through the system that will stop the bubbles from popping back whenever the flow stops. There are too many kinks and bends in the brake plumbing to make life easy, and the 1250 seems to be even more tricky than the 1200...

An alternative is to use a syring and push the fluids from the caliper and up, but then you need to drain the reservoir and find a way to avoid leaking from the bleed nipple, in short a messy option.

I never thought of lifting the bike, 10 points for ingenuity

 

[ShakeyGS]

Where does the compressor driven system put the fluid in? I assume the reservoir but how does it seal in that case?

 

[m1keyp]

Just fit Speed Bleeders

Goodridge Speed Bleeder
Thread Size: 8 x 1.25mm
GDRBN508PSB £12.72 1 1 £12.72
Goodridge Speed Bleeder
Thread Size: 6 x 1mm
GDRBN561PSB £11.71 1 1 £11.71
Goodridge Speed Bleeder
Thread Size: 10 x 1mm
GDRBN510PSB £12.72 1 1 £12.72

Demon Tweeks are still shipping

 

[ShakeyGS]

Thanks M1key. I’ve been thinking along those lines too. Those are a lot cheaper than e.g. motorworks.

I note that the nipple on the front left calliper seems to be made to fit some kind of fiendish BMW service device to.

 

[knutk]

Where does the compressor driven system put the fluid in? I assume the reservoir but how does it seal in that case?

You connect the bleeder tube to the nipple on the caliper, the lid on the reservoir must be off. Open up for the continuous suction on the bleeder, then you open up the bleed nipple. There will be air leakage along the threads of the nipple. However as the suction is on, no air will enter the system, and the spillage on the bike will be an absolute minimum.
You leave the suction on, and adjust the flowrate through the system by turning the bleed nipple. While it bleeds, you have a bottle with fresh brake fluid available and keep refilling the reservoir as the level drops.
As the fluid flows downwards, the better the flow is, the easier it is to allow for the bubbles to go with the flow.

You will not be able to observe whether there is air in the fluid by looking at the bleed hose, as the hose also draws air from the nipple. However, in my experience 2 - 3 refills of the reservoir is enough. Also, prior to the bleeding, I suck out all the old fluid from the reservoir and refill it with fresh fluid.

This procedure har worked fine on every bike I've ever owned, until I got the 1250. On the 1250, after all the old fluid is replaced, the brakes in front where still spongy, while the rear wheel breake was good. Leaving the bike overnight with the brake lever Zip-tied to the handlebar took care of the rest, so the next morning, the brakes where fine.

 

[knutk]

Just fit Speed Bleeders

Goodridge Speed Bleeder
Thread Size: 8 x 1.25mm
GDRBN508PSB £12.72 1 1 £12.72
Goodridge Speed Bleeder
Thread Size: 6 x 1mm
GDRBN561PSB £11.71 1 1 £11.71
Goodridge Speed Bleeder
Thread Size: 10 x 1mm
GDRBN510PSB £12.72 1 1 £12.72

Demon Tweeks are still shipping

Speed bleeder is a smart device. However, with all the kinks in the brakelines, in order to get the air out you need a continuous flow of fluids. Once the flow stops, the bubbles pops back to where they where, therefor you need a compressor driven bleeder.

As you have the compressor driven bleeder anyway, and allow for suction to start prior to open up the bleed nipple, the speed bleeder serves no purpose.

 

[ShakeyGS]

Thanks knutk. I guess the bleeder must have some kind of venturi to be able to generate suction from positive pressure.

I have ordered the speed bleeders in the hope that their engineering tolerances are better than the BMW ones. Once I take the velcro strap of the front brake later today, I’ll decide whether or not to look for a pressurised bleeder.

 

[CPJS]

Where does the compressor driven system put the fluid in? I assume the reservoir but how does it seal in that case?

Get yourself a spare reservoir cap, drill it and used one of these.

https://www.halfords.com/tools/hand-tools/automotive-tools/gunson-eezibleed-321158.html

I have hav one of these for years, although I have not needed to use it on the GS, I have used it on lots of previous bikes and cars.
If no suitable reservoir cap was available I connect it up to a bleed nipple and reverse bleed it back up into the reservoir. If doing it this way don't use too much pressure or the tube will blow off the nipple, keep an eye on the reservoir to make sure it doesn't overflow.
Always start with the pads pushed right back in the calipers to reduce the liquid volume in the system.

 

[ShakeyGS]

I used to own one of those Gunsons gadgets but I just couldn't get it to work. Dunno what happened to it.

 

[knutk]

Thanks knutk. I guess the bleeder must have some kind of venturi to be able to generate suction from positive pressure.

I have ordered the speed bleeders in the hope that their engineering tolerances are better than the BMW ones. Once I take the velcro strap of the front brake later today, I’ll decide whether or not to look for a pressurised bleeder.

That is correct, a venturi creates low pressure, hence you get a continous motion on the fluid that moves through the brake system, bringing any bubble along on the way.

I have no experience with a speed bleeder beyond understanding how it works. Depending how firm the spring inside the speedbleeder is, I see a potential problem where the checkvalve will block the flow due to too low suction from the vacuum bleeder. I may be wrong here, perhaps someone out here has experience with the combination of suction bleeding and speedbleeder?

 

[pistole]

Am experiencing same loss of rear brake pedal after repeated bleedings.

Put in a new master cylinder, same result.

Am currently trying out ATE200 high temp brake fluid.

I suspect it's a failure in the abs cum proportioning valve (ie the linked brakes). Try bleeding and then test riding using only the rear brake pedal, the brake will not deteriorate.

With bike on center stand, can you "induce" linking by braking the front and seeing whether the rear wheel is stopped? I can occasionally get this to happen, almost randomly, hence my guess about the failure of proportioning valving.

Nonetheless, am mystified as to where this air is getting into the system. Has there been any conclusions yet here?

Thanks

 

[ShakeyGS]

Not yet. I’m waiting for my new toys to arrive.

 

[ShakeyGS]

Plan of attack for front brake system;
1. Wait for new compressor powered vacuum bleeder to arrive.
2. Attach same to a speed bleed nipple to see if it will draw fluid from a jar
3. Use result to decide whether or not to fit speed bleeders
4. Bleed front using vacuum bleeder.

But.........

The speed bleeders arrived two days ago and the vacuum bleeder hasn’t yet........... hmmmm.

Fukkit! I need something to do. Let’s play with the speed bleeders anyway

 

[pistole]

It's quite easy to bleed it the old fashioned way. Why are you looking at speedbleeders?

 

[ShakeyGS]

It's quite easy to bleed it the old fashioned way. Why are you looking at speedbleeders?

Because I’m not getting the results I want and because I like messing about in the garage.

 

[knutk]

It's quite easy to bleed it the old fashioned way. Why are you looking at speedbleeders?

It is indeed easy to bleed in the old fashioned way.
But in my experience, with ABS equipped bikes, any time you get some air into the system it's mighty hard to get the air out due to all kinks and bends in the system. One pump will indeed move the potential bubble, but once the motion of the fluid stops, the bubble pops back.
Besides, leakage around the bleed nipple makes it a bit messy.

Suction bleed is easy to do alone, no mess around the bleed nipple and way faster to do.

 

[Alan R]

It's quite easy to bleed it the old fashioned way. Why are you looking at speedbleeders?

Speed bleeders are useful because it allows one to push through quickly a large volume of fluid. If there are air bubbles present between the high reservoir and the low caliper, if one is not quick enough, the pause while the normal bleed screw is closed allows the fluid to be momentarily stilled and the air can percolate back up the hose, therefore never reaching the caliper bleed screw. All the ABS gubbins presents a high point between the reservoir and caliper where bubbles can accumulate: push fluid through fast in a continuous flow and they are swept to the caliper bleed point.
Alan R

 

[pistole]

thanks for the replies @ knutk & Alan R.

am not arguing but trying to understand/resolve my missing rear brake. If I get the drift of the last couple of posts here , the suspicion is that air is somehow being trapped at a level above the master cylinder and the caliper, in other words at or near the ABS unit ? And when bleeding normally , we can't chase all the air bubbles out of the circuit because the air bubbles have the opportunity to recover and make their way up between strokes of the brake pedal ?

(1) is this even an acknowledged theory ? It seems crazy that the bubbles can migrate so quickly back up the miniscule passage in the brake hose in between rear pedal strokes to escape being expelled at the caliper. That and every failed rear brake which is bled is accompanied by loads of air being expelled from the caliper pointing to the fact that air is somehow being generated and duly expelled during the bleed but somehow during the bike's use , air is generated in the rear circuit.

(2) I understand that the highest point in the rear brake circuit is the ABS unit. That is the same for the front circuit as well. If so , why are we only experiencing issues with the rear circuit and not the front?

(3) does BMW even recommend a pressure / vacuum bleeding procedure ? My understanding of the service manual makes no mention of any pressure/vacuum system.

(4) has any suspicion fallen on the abs unit / pressure modulator cum linking valve ?

(5) why does my rear pedal remain firm when I ride only using the rear brake pedal and avoid use of the front ?

.

as a separate note , I am testing high temp brake fluid now (ATE200) and my results have been positive. Will keep you guys posted. thanks again.

 

[CPJS]

Because I’m not getting the results I want and because I like messing about in the garage.

Get yourself plenty of brake fluid as when using a pressure bleeding system the stuff flies through the system. Just a hose on the normal bleed nipples works well.

 

[ShakeyGS]

I hope 2 litres will be enough

I had a play with my speed bleeders using my hand vacuum pump and got very mixed results between them as to how much vacuum was required for them to draw air through and how far they would relieve the vacuum before closing again. Utterly pointless exercise but it killed some time. I haven’t fitted them yet.