Brakes again .......

[ShakeyGS]

In order to follow the BMW method for changing my brake fluid, I’ve attached a hand operated vacuum pump to the bleed nipple on the rear caliper but I find that as soon as the nipple is open wide enough to let fluid out, it just sucks air in around the threads.

Has anyone found a way to make a vacuum pump work? Should I maybe fit a speed bleed nipple or wrap the existing one in ptfe tape?

 

[TheStammeringBiker]

Ive found its best to “finish the job by hand” so to speak as I’ve always had this issue using my Mityvac. Personally I only ever use the Mityvac now if I’m completely emptying and filling a system.

I’ve heard the PTFE tape method works though.

 

[ShakeyGS]

Ive found its best to “finish the job by hand” so to speak as I’ve always had this issue using my Mityvac. Personally I only ever use the Mityvac now if I’m completely emptying and filling a system.

I’ve heard the PTFE tape method works though.

Agreed. I was trying to empty the rear brake circuit.

 

[R90CPN]

So long as you pull enough fluid through to completely replenish the fluid in the system pulling air in around the treads will not introduce any air in the system. Just pull plenty through until the fluid in the bleeder is nice and clear and close the nipple whilst still pulling positive pressure from the bleeder. PTFE or grease around the thread will help but there is no need really.

 

[ShakeyGS]

So long as you pull enough fluid through to completely replenish the fluid in the system pulling air in around the treads will not introduce any air in the system. Just pull plenty through until the fluid in the bleeder is nice and clear and close the nipple whilst still pulling positive pressure from the bleeder. PTFE or grease around the thread will help but there is no need really.

It’s pulling in so much air it’s not shifting the fluid.

 

[CPJS]

It’s pulling in so much air it’s not shifting the fluid.

If air is finding it's way into the system so as to require bleeding I would not recommend a vacuum system to do this. A negative pressure method will be trying to pull air in on the whole system, you may make whatever the braking systems problem is worse.
It might be worth trying a positive pressure type. I have an old Gunnisons one that attaches to a tyre valve and uses that pressure to push the fluid through.

 

[SPL]

It’s pulling in so much air it’s not shifting the fluid.

Shakey - my MightyVac will do this if

I get carried away and pump in too much vacuum (after forgetting how many times you're supposed to pull the trigger and refusing to RTFM)
and
I use the wrong size connector tube for the bleed nipple (so it's just a little but loose)
or
I don't quite back off the nut enough to let the fluid flow properly

Could it be any of the above??

 

[ShakeyGS]

I’ve ordered a box of O rings. I’m going to try to put one over the bleed nipple threads and after opening the nipple, push it down towards the caliper to try and seal the threads a bit. I’ll report back.

 

[knutk]

It’s pulling in so much air it’s not shifting the fluid.

The clue is to use a compressor driven pump, such as this one:
https://www.amazon.com/Mityvac-6830-Brake-Bleeder/dp/B000JFN9WC/ref=sr_1_16?crid=XYYPFKLREJCE&dchild=1&keywords=vacuum+bleeder&qid=1587415284&sprefix=vacuum+bleeder%2Caps%2C240&sr=8-16

It will still pull air, so you can not establish whether there is air or not in the fluid being sucked out. Hence the BMW comment about finishing the job with 'old fashioned' bleeding.

Prior to the 1250, the suction bleeding worked fine on all bikes in my ownership. For the 1250, it still puls the fluids out in the same manner, but even so, by the time I'm done, the front brake lever is stil spongy, the rear brake works just fine.

So, for my 1250 GSA, after bleeding the brakes, I put it on center stand and then I zip tie the brake lever to maximum pressure (which means pulling the lever until it hits the grip) and leave it over night. Next morning the brake feels just fine.

 

[ShakeyGS]

Well, the O ring didn’t perform as I'd hoped so I reverted to using PTFE tape. I can’t believe what a sloppy fit the bleed nipple is to the calliper! Or how tricky it is the tape it whilst keeping a finger on the hole.
Anyway, this did the trick and I was easily able to drain the system. There’s definitely air in there now .

Having used the vac to slowly draw new fluid through and then reverting to my usual no-return bleeder, I can get a reasonable feel on the pedal for a short while and then it just goes to full travel. Next step is to exercise the ABS modulator with the GS911. This thing ain't gonna beat me !

 

[ShakeyGS]

I’ve now done all that and no additional air came out of the system. However, the pedal still feels a bit spongy so I looked at the realtime values on my GS911. The pressure controller for the rear shows a starting value of -1.0 barg (eh?) and the brake circuit 0.0barg.

On depressing the brake pedal, the circuit goes up to 32 barg which sound OK to me. The controller circuit also goes high but I didn’t note the value.

Hmmm.

I’m getting paranoid now. I think I might put the bike on the sky lift nose down so as to put a gradient on the line from the ABS modulator just in case that teaspoon of volume is actually trapped air.

 

[CPJS]

Well, the O ring didn’t perform as I'd hoped so I reverted to using PTFE tape. I can’t believe what a sloppy fit the bleed nipple is to the calliper! Or how tricky it is the tape it whilst keeping a finger on the hole.
Anyway, this did the trick and I was easily able to drain the system. There’s definitely air in there now .

Having used the vac to slowly draw new fluid through and then reverting to my usual no-return bleeder, I can get a reasonable feel on the pedal for a short while and then it just goes to full travel. Next step is to exercise the ABS modulator with the GS911. This thing ain't gonna beat me !

I think that most of the spare movement in the lever is not air but 'slack' in the workings of the linked braking/abs module. My rear brake works fine with no sponginess but a lot of travel till the 'slack' is taken up. When riding if I apply the front brake on lightly then apply the rear that slack has been taken up, then the rear brake feels like it is supposed to.

 

[GORILA]

Why don't you keep the brake pedal depressed overnight, with a brick or other heavy item. That usually helps 'firming up' the brake!

 

[GSBruce]

I use teflon tape around the nipple plus a very small zip tie as a hose clamp to tighten the bleed hose around the nipple. Only loosen the bleed nipple enough to allow flow. As stated above, as long as you're pulling a vaccum and keep fluid in the master cylinder reservoir, you'll be fine. The bubbles in the suction hose are all coming in downstream of the brake system.

 

[ShakeyGS]

4kg kettlebell hanging right now


Why does this work?

 

[GSBruce]

4kg kettlebell hanging right now


Why does this work?

This is Boyle's Law and gravity in action. By pressurizing the brake fluid any air bubbles which may be too large to fit in the brake line under atmospheric pressure are compressed and become smaller bubbles. If the small bubbles are then small enough to fit in the brake line they will slowly travel upward and ultimately make their way out of the brake line and into the master cylinder and then you'll have only brake fluid in the system downstream of the master cylinder. I only know of this working on front brakes where the master cylinder is somewhat higher than the brake cylinder and the fluid path is all uphill from the brake cylinder to the master cylinder. Tne rear brake system path from brake cylinder to master cylinder seems more like an up and down situation to me and I doubt the "leaving the rear brake system pressurized overnight" method will work under that condition - the bubbles of air will be trapped at the high point(s) in the system - they will never travel downhill in a static situation.

 

[M30SJM]

This is Boyle's Law and gravity in action. By pressurizing the brake fluid any air bubbles which may be too large to fit in the brake line under atmospheric pressure are compressed and become smaller bubbles. If the small bubbles are then small enough to fit in the brake line they will slowly travel upward and ultimately make their way out of the brake line and into the master cylinder and then you'll have only brake fluid in the system downstream of the master cylinder. I only know of this working on front brakes where the master cylinder is somewhat higher than the brake cylinder and the fluid path is all uphill from the brake cylinder to the master cylinder. Tne rear brake system path from brake cylinder to master cylinder seems more like an up and down situation to me and I doubt the "leaving the rear brake system pressurized overnight" method will work under that condition - the bubbles of air will be trapped at the high point(s) in the system - they will never travel downhill in a static situation.

Ive never understood this part, as when the lever is pulled, the master cylinder reservoir and pressure line are no longer connected, separated by the piston. Or is it that the air travels to the top, then when you release the lever the air can then rise into the reservoir?
Stu

 

[M30SJM]

Well, the O ring didn’t perform as I'd hoped so I reverted to using PTFE tape. I can’t believe what a sloppy fit the bleed nipple is to the calliper! Or how tricky it is the tape it whilst keeping a finger on the hole.
Anyway, this did the trick and I was easily able to drain the system. There’s definitely air in there now .

Having used the vac to slowly draw new fluid through and then reverting to my usual no-return bleeder, I can get a reasonable feel on the pedal for a short while and then it just goes to full travel. Next step is to exercise the ABS modulator with the GS911. This thing ain't gonna beat me !

You could put an o-ring under the bleed nipple, you wouldnt be able to tighten it and close the system without removing it, but it might help you to get the fluid moving.

 

[ShakeyGS]

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

 

[ShakeyGS]

Ive never understood this part, as when the lever is pulled, the master cylinder reservoir and pressure line are no longer connected, separated by the piston. Or is it that the air travels to the top, then when you release the lever the air can then rise into the reservoir?
Stu

The second part