Rear control circuit bleeding

[Hedgry]

Today I went to rebleed my control circuit. Last time I did it-a few weeks ago- I didn't have info on the proper order for bleeding the circuits. You guys graciously provided that so I figured I'd do it over just in case...here's what seemed to go wrong: Bled the front control circuit, no problem. Went to bleed the rear control circuit and I could not get the brake fluid to run clear of bubbles on the #1 and #3 circuits. I went through two pints of fluid trying. The bubble were very very small for the most part. I never let the reservoir run too low. I finally gave up. It would seem to me that if air was getting sucked in somewhere it would have to be at the rear master cylinder and if it was you'd think it would be leaking when under pressure? I inspected the entire rear circuit and could detect no leaks anywhere. Was air just getting sucked in from behind the bleed screw when I had them open giving me a false reading? The foot brake doesn't feel spongy and the brakes are plenty strong when just employing the rear lever...both wheel's brakes apply, doesn't take much to start a lock up. The servos sound exactly the same with front or rear applied. Am I losing it? Any thoughts?

Thanks,

Henry

[KCB]

Hedgry,

your issue of extremely small bubbles in the bled fluid is not uncommon, especially if you are using a vacuum pump bleeder like a MightyVac. If you have "foam" type bubbles 1/64"(think stick pin hole in a piece of paper) or smaller you do not have an air issue.

Sometimes this condition develops from the bleeder adapters not fitting perfectly.

This size will dissipate all on their own, keep an eye on the reservoir level as a backup. Have a beer you did the hard part well!!

[philbytx]

Did you try "barely" cracking the bleeder(s) open, so that it is barely allowing fluid through into the tube?

I had the same issue and AFAIAC it appears to be air introduced between the bleeder and the controller where it screws in. I did the same as you and just buttoned it down and used it. Nary a problem with 4 bleeds so far

[Hedgry]

Whew! That's what I was hoping to hear!!!! Thanks so much. I did try barely cracking the bleeders, it seemed about the same but of course the flow was greatly reduced so I was starting to wonder if I was just seeing things. I wasn't using a vacuum bleeder, just doing it old school. This is a great releif. I was worried sick about air in the circuit damaging the control module-but I think that would be more of an issue on the brake control side, right? Cavitation of the servo pumps? Beer will be drunk and bike will be ridden (beer tonight, bike tomorrow of course!).

Henry

[AndyS]

HI Hedgry, had the same problem and got through litres of fluid trying to get to the bottom of it. I never did.

I was hoping to find thread sealer similar to the type that speedbleeders put on their bleed adaptors so that it would get rid of the air path. unfortunately I could not source any of the stuff in the UK.

DON'T USE PTFE TAPE.

I am still dissapointed in the fit of the adaptors if it is such a common problem.

 

Andy

[DrPaul]

Since brake bleeder valves rely on their tip bottoming out in the bore to seal the system, the threads are machined with fairly wide tolerances - even a turn or two open will permit those "tiny bubbles" to enter around the valve and appear back through your bleeding tubing. I imagine we've all fallen for that at one time or another. Smear some light grease around the bleeder base if you don't want to see them again! Also, unless you introduce air into the control systems, you needn't mess with bleeding at all three points on the ABS unit. Just do #3 on each side.

[AndyS]

Since brake bleeder valves rely on their tip bottoming out in the bore to seal the system, the threads are machined with fairly wide tolerances - even a turn or two open will permit those "tiny bubbles" to enter around the valve and appear back through your bleeding tubing. I imagine we've all fallen for that at one time or another. Smear some light grease around the bleeder base if you don't want to see them again! Also, unless you introduce air into the control systems, you needn't mess with bleeding at all three points on the ABS unit. Just do #3 on each side.

 

Sorry to disagree Dr Paul, but:

 

What has the tip bottoming into the bore to seal got to do with a 'loose thread tolerance'? A close tolerance is just as good if the thread is concentric to the tapered base (which it will be).

 

I would argure that if anything - the design intent for hydraulic being kept seperate from air - the Design Intent would be to have close tolerances.

 

As for applying grease to the threads -BE VERY CAREFUL - we are into the realms of contamination here. What grease are you recommending?

 

Your last observation (about only needing to bleed #3 nipple on each circuit)is line with BMW service bulletin 34 002 06 (019), but I would put odds on that if you did that, and then went back to the others, you would get some air out.

 

Andy

[DrPaul]

Sorry to disagree Dr Paul, but: (a) What has the tip bottoming into the bore to seal got to do with a 'loose thread tolerance'? A close tolerance is just as good if the thread is concentric to the tapered base (which it will be). I would argure that if anything - the design intent for hydraulic being kept seperate from air - the Design Intent would be to have close tolerances. (b) As for applying grease to the threads -BE VERY CAREFUL - we are into the realms of contamination here. What grease are you recommending? © Your last observation (about only needing to bleed #3 nipple on each circuit)is line with BMW service bulletin 34 002 06 (019), but I would put odds on that if you did that, and then went back to the others, you would get some air out. Andy

a) If the tolerances were very close (fine machine thread pitch), little if any air would leak past the threads and we wouldn't be having this discussion.

b) Vaseline (petrolatum to you Brits) applied sparingly to the caliper surface around the bleeder to act as a seal.

c) Only if air was inadvertently introduced into the system; where else would it come from?

[AndyS]

a) If the tolerances were very close (fine machine thread pitch), little if any air would leak past the threads and we wouldn't be having this discussion.

b) Vaseline (petrolatum to you Brits) applied sparingly to the caliper surface around the bleeder to act as a seal.

c) Only if air was inadvertently introduced into the system; where else would it come from?

 

a/. That's my point. They ought to be close, but unfortunately often they are not.

 

b/. Not sure but hopefully someone else will confirm if this acts as an inert sealer or a contaminant.

 

c/. Air will get potentially trapped at any riser. As air is pushed through the system (because we previously emptied it out) it can get caught in any of them, each of the blled ports on the servo can act as a riser and so there is potential for air to get 'stuck' there. The option of bleeding this out has to be a good thing.

 

Andy

[DrPaul]

a) If the tolerances were very close (fine machine thread pitch), little if any air would leak past the threads and we wouldn't be having this discussion.

b) Vaseline (petrolatum to you Brits) applied sparingly to the caliper surface around the bleeder to act as a seal.

c) Only if air was inadvertently introduced into the system; where else would it come from?

 

a/. That's my point. They ought to be close, but unfortunately often they are not.

 

b/. Not sure but hopefully someone else will confirm if this acts as an inert sealer or a contaminant.

 

c/. Air will get potentially trapped at any riser. As air is pushed through the system (because we previously emptied it out) it can get caught in any of them, each of the blled ports on the servo can act as a riser and so there is potential for air to get 'stuck' there. The option of bleeding this out has to be a good thing.

 

Andy

Ok, Andy, we've both made our points. It's probably time to let this thread die with dignity. Take care.

[AndyS]

Yes, I'm sorry for all the readers too, who have seen this question go off at a tangent.

 

Hedgry, I do hope your brakes are OK now.

 

Keep well

 

Andy