Author Topic: Air in hydraulic brake switch?  (Read 229 times)

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Offline The Lone Builder

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Air in hydraulic brake switch?
« on: October 26, 2018, 06:09:05 am »
I've been battling with my front brake, trying to get it to stiffen up. I think, after trying every trick mentioned on forum, that I have it as good as it's gonna get.

The last frustration is that the front brake switch is not working. It was working previously - just after fitting - and the wiring is OK.

Could it be that there is a pocket of air stuck in the switch that is stopping it from working? If so how can I try to get this out without introducing more air into the system?

I thought of just using the switch as a bleeding point, but as y'all know, the switch is not exactly accessible and this approach will spill brake fluid all over the lower yoke.

Cheers

Sean
CB750 K2 - A work in progress
CB750 K1 - The less, said the better!
CB450 K1 - It's black.
CB400F Supersport - It's rusty!

Offline Henning

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Re: Air in hydraulic brake switch?
« Reply #1 on: October 26, 2018, 06:45:19 am »
The switch has most likely passed on, I doubt it has anything to do with air in the system so only option is replacement. Have had this happen to me before. When you get it off you can poke a matchstick down the hole at the back to see if you can get it to activate.
Re. your soggy front brake lever, I can relate that recently I replaced the caliper on mine with a good used one from a member here. The brake bled right up and I've never had a firmer brake lever. Go figure, I did exactly what I've done half a dozen times before and the result is just better, I didn't touch anything but the caliper leading me to think there was something wrong with the old one.

P.S. Enjoying what you are doing down there, I'm from Cape Town myself, now resident in Denmark.
71 or thereabouts 750 K1 - this one should have been put down

Offline evanphi

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Re: Air in hydraulic brake switch?
« Reply #2 on: October 26, 2018, 06:53:08 am »
Did you do the whole "bike on sidestand bars locked left lever ziptied half-back over night" trick?

Give that switch a few taps with a rubber mallet or screwdriver handle and it should pop the air bubbles out with the bike in that position. It should no affect the switch's operation.

Have you checked the switch for continuity with a voltmeter when you squeeze the brake lever?
--Evan

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She's a mix-matched (former) basket case, but she's mine.

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Offline Kevin D

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Re: Air in hydraulic brake switch?
« Reply #3 on: October 26, 2018, 08:20:17 am »
Quote
The last frustration is that the front brake switch is not working. It was working previously - just after fitting - and the wiring is OK.

Lone Builder, I had a similar problem and it turned out to be broken wires in the harness. The copper strands were broken inside the plastic jacket: the wires looked ok, worked ok sometimes, but tested bad when flexing the brake light wires.
 A couple butt splices and some super flex wire and it has been good for 15? years.

I have also had success with evanphi's tie the lever back method.

...and, nice photos at the lake. Extra quiet places are super.
71 CB750 K1
85000 mi
Original Owner

Offline The Lone Builder

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Re: Air in hydraulic brake switch?
« Reply #4 on: October 26, 2018, 12:38:57 pm »
The switch has most likely passed on, I doubt it has anything to do with air in the system so only option is replacement. Have had this happen to me before. When you get it off you can poke a matchstick down the hole at the back to see if you can get it to activate.
Re. your soggy front brake lever, I can relate that recently I replaced the caliper on mine with a good used one from a member here. The brake bled right up and I've never had a firmer brake lever. Go figure, I did exactly what I've done half a dozen times before and the result is just better, I didn't touch anything but the caliper leading me to think there was something wrong with the old one.

P.S. Enjoying what you are doing down there, I'm from Cape Town myself, now resident in Denmark.

The switch, Henning, is a new one; I used it instead of the original during the rebuild. Maybe I'll try the old one, if I can find it. I was also thinking of trying a different capiler, but the old pad/cylinder is stuck in there. The weather here is still better!  ;D

Did you do the whole "bike on sidestand bars locked left lever ziptied half-back over night" trick?

Give that switch a few taps with a rubber mallet or screwdriver handle and it should pop the air bubbles out with the bike in that position. It should no affect the switch's operation.

Have you checked the switch for continuity with a voltmeter when you squeeze the brake lever?

Haven't tried it with the bike on side stand, but I've done everything else. Will give it a go.


Lone Builder, I had a similar problem and it turned out to be broken wires in the harness. The copper strands were broken inside the plastic jacket: the wires looked ok, worked ok sometimes, but tested bad when flexing the brake light wires.
 A couple butt splices and some super flex wire and it has been good for 15? years.

I have also had success with evanphi's tie the lever back method.

...and, nice photos at the lake. Extra quiet places are super.

The wiring is Ok, Kevin. Jumping the connections lights up the light. The harness has been "sorted" during the rebuild.

Seventeenth century French scientist Blaise Pascal is perhaps best remembered for his contributions to the field of pure geometry. In the 39 years that he lived, he found time to invent such modern day fundamentals as the syringe, the hydraulic press, and the first digital calculator. And, if that weren't enough, he was also a profound philosopher. He said,

"All man's miseries derive from not being able to sit quietly in a room alone."


Perhaps if he had a lake to sit beside ...

Sean


CB750 K2 - A work in progress
CB750 K1 - The less, said the better!
CB450 K1 - It's black.
CB400F Supersport - It's rusty!

Offline ADW

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Re: Air in hydraulic brake switch?
« Reply #5 on: October 26, 2018, 01:20:31 pm »
I would suggest removing the switch and using a syringe, eyedropper, something like that to fill the switch with brake fluid BEFORE you screw it on. The surface tension of the brake fluid should hold it into the switch if you're quick enough on the install. If you installed the switch dry it may not have any capillary action to help "pull in the fluid" and "let out the air bubble".

Offline Bodi

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Re: Air in hydraulic brake switch?
« Reply #6 on: October 27, 2018, 07:00:58 am »
There is no magic in brake fluid, the switch works on pressure whether air or fluid pressure. Air in there will contribute to a mushy lever feel but can not affect the switch function.
Either the switch or its harness wiring is causing your problem.

Offline bryanj

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Re: Air in hydraulic brake switch?
« Reply #7 on: October 28, 2018, 03:03:43 am »
Just because it is new does not mean it is any good. Spent hours chasing faults caused by "new" parts
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

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Offline The Lone Builder

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Re: Air in hydraulic brake switch?
« Reply #8 on: October 29, 2018, 02:34:26 am »
Ha!

Well now; yesterday, before reading this, I did exactly what ADW suggested - great minds and all that  :). While I had it out, I checked the operation of the switch using a 3mm allen key and it seems to be working. I attached it to the wiring and tried again with the allen key; the stop light lit up fine, so, as I said earlier, the wiring is OK and it would seem the switch is too.

However when I reinstalled it ....... no difference  :(  No light and lever still a bit soft. Stupidly, I forgot to put the washer on when reinstalling; woudl this make any difference?  - no leaks.

Now a question. Has anyone taken one of these switches apart? I was using a syringe with a hypodermic needle and it felt as if there was a rubber "something" in there. It didn't cover the comlpete channel; it seemed to be on one side only.
CB750 K2 - A work in progress
CB750 K1 - The less, said the better!
CB450 K1 - It's black.
CB400F Supersport - It's rusty!

Offline Henning

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Re: Air in hydraulic brake switch?
« Reply #9 on: October 29, 2018, 10:20:45 am »
There will be a diaphragm of sorts in there with brake fluid on the one side and air on the other. If the diaphragm is broken and there's fluid on both sides it won't work, but it'll still be possible to activate the switch with your Allen key. Like I said, it's gone south  ::) Life's too short to spend time trying to repair something like that.
71 or thereabouts 750 K1 - this one should have been put down

Offline The Lone Builder

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Re: Air in hydraulic brake switch?
« Reply #10 on: November 08, 2018, 03:06:49 am »
Thanks.

Just got a new one. Will fit it over the weekend.

CB750 K2 - A work in progress
CB750 K1 - The less, said the better!
CB450 K1 - It's black.
CB400F Supersport - It's rusty!

 

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