Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Gromitdog on September 08, 2020, 12:00:39 PM
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Please could anyone shed any light why it is so difficult to bleed the front brake system on a CB750 K8 use import :( :( ?
I have on Graham and Julies advice brought a S/Steel piston, new master seal kit, new s/steel braided hose which on my model is one piece which is 80cm in length ALL THE SEALS are genuine Honda seals using Dot 4 fluid put in new crush washers at the master cylinder banjo joint
1) I have tried bleeding it by the normal std method of opening and closing the lower bleed nipple
2) Using a syringe I have back bleed it by pushing the build towards the master cylinder
3) Using a syringe I have bled it by sucking the fluid through from the master cylinder
4) Without out the lower bled nipple being open & with constant small movement of the brake lever for 20 minutes.
Comparing the movement of the front brake lever with my CB550K3 My 550 moves back less than half way and then unable to pull it back any further ........ With my 750 I am able to pull back the brake lever back to within 1 finger thickness of the handle bar most times And if i really pull the brake lever hard I can get it back to the handle bar
which i am not happy with ( Not got the confidence in it stopping me )
Any help or guidance why I am having so much trouble with this simple task
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On my 500 I use a one way bleed tube that I’ve had for decades which I think helps. That may not be the issue but are you using a one way tube from the nipple to the low level container?
Also I didn’t need to but I wonder about putting the bike on the side stand and turning the bars to the left to create a slope to maybe help bubbles up to the master cylinder.
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Something I learned when having trouble bleeding my CB750 front brake was that pulling the lever all the way back to the bar is very wrong, as your taking the piston to where it wasn't ever meant to go.
Either my shop manual or a service bulletin has a picture of something around the size of a matchbox taped to the bar to limit the travel.
You may already know this, but worth pointing out.
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I do mine the same ways as Jeremy with the bike on the side stand and the bars in the highest position, mine is also fitted with speed bleeders.
Cheers
Dennis
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Before panicking further, I assume you have adjusted the caliper correctly, as in a 6 thou clearance between the fixed pad and the disc,,? If not, excess clearance there will have you chasing your tail forever.
Sent from my POT-LX1 using Tapatalk
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On most of my cars I've used a Lockheed Easybleed (LE) using air from a spare wheel to power it. I've always got a decent pedal by this method. I also like the look of the one Ed China uses that sucks fluid out of the bleed nipple.
Later design master cylinders often can't work with LE. I might source a suction type for future use.
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Are you confident the internal routing from master cylinder pot into the piston are scrupulously clean, also not compromised for diameter ( part blocked) ?
It sounds, if all the air is out of system, that the fluid may not be filling the piston void competently. This could result in the increase in travel you are getting.
Try this, ( not bleeding but with it fully operational) hold the bar with left hand and wack the lever in very quick succession quite forcefully, about six times but holding the last push, does the level stop further out with that last push? In old fashioned terms :) does it improve pressure and "pump up".
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I had terrible trouble bleeding my K2 when I built it - tried everything but turned out to be a blocked hole in the master cylinder - there's 2 holes, one smaller than the other. When I completely cleared the smaller one, it bled 1st time. Just done same on a K6 I've built and it too bled 1st time. From memory, it was 1mm and cleaned out with 1mm drill
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Hi All
Thank you all for coming up with good points on what may be causing the problem in bleeding my brakes I think i will start with the couple of different ways to bleed the system that have been suggested ie side stand and easy bleed then if no joy I am going to strip down the master cylinder and check the seals etc. And go through the setup again just cant believe how difficult it is to bled >:( >:( such a small system
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I can't understand why so many people have trouble bleeding the system. If the M/C is clean and the new kit assembled correctly, the caliper totally cleaned it, should bleed straight away.
If not then there is something wrong.
You can not compare to other systems by the lever travel. I have one 750 that the lever only comes back half a inch and the pads are locked on the disk, another 750 the lever travels a lot more before the brake is locked on. Though they have both the same system and new parts fitted. In the trade we call this " the characteristic of the vehicle ;)
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Thank you everyone who gave me advice & suggestions on how to bleed the front brake on my CB750. I followed the advice given and after fitting a new bleed nipple and following the advice on the forum it has improved so much and now is closer to where I happier with it, Possibly now it just need the brake pads bedding in.
Thank you all for your time help and guidance with this small problem
Gromitdog
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An important one though! The most really.
My EBC semi sintered pads took a good 2-300 miles to bed in probably due to gentler riding these days, and pretty potent now given the technology, just given a bedded in adjustment.
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Another tip I was given which works is, pull the leaver back put quite a bit of pressure on it and tape it in place.
Leave it like this for a couple of days tapping the flexible hoses every now and again.
Always improves the leaver travel ;) :D
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Another tip I was given which works is, pull the leaver back put quite a bit of pressure on it and tape it in place.
Leave it like this for a couple of days tapping the flexible hoses every now and again.
Always improves the leaver travel ;) :D
One hour is enough ;)
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If you remove the banjo at the master cylinder, put your finger over the hole, pump the lever, you can establish if it works or not.
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PUT GLOVES ON if you are using a finget