Brakes won't bleed - Honda Motorcycles - FireBlades.org
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post #1 of 9 Old 05-29-2016, 2:56 PM Thread Starter
 
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Brakes won't bleed

Hi there,

I feel like I've done a really stupid thing, Basically, I have replaced the brake seals all round and all the brake hoses and the front master cylinder I have overhauled, and now the brakes just will not bleed. Back brake came up a treat, but the fronts nothing. I spent over 5 hours trying to bleed them and air just keeps coming out.

I am starting to suspect now that the master cylinder might be at fault and sucking fluid back in after it is pumped and that the air I am seeing in the bleed tube is actually coming from the threads of the bleed nipple.

Is there something in the master cylinder that I have neglected, like some sort of hole like a breather that I have missed that stops the fluid being sucked back into the mastercylinder from the brake hoses?

Any help with this would be very much appreciated, I have one day to fix this issue before I have to buy another vehicle to get to work on on tuesday.

Kind regards

Rob
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post #2 of 9 Old 05-29-2016, 3:22 PM
 
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Re: Brakes won't bleed

5 hours bleeding time? Wow. After 5 minutes I'm done. This is what I'd do. I remove the banjo bolt and get a the same metric bolt and put it where the banjo bolt was, starting right at the top outlet to your master cylinder, and then bleed it to see if it's even doing it's job. If you can bleed that, then you know your master cylinder is working, and if you can't, you know the master cylinder is damaged, or your assembly was wrong, or the kit was the wrong kit.

If you can bleed it, move that bolt further down the line isolating each side of your brake lines (left side, right side) bleeding from the caliper to isolate where you have a problem, but I'm sure when you start at the master cylinder you'll identify it is right there. Sometimes old master cylinders have worn cylinders or they get pitted, so a replacement is the only answer. Kits don't always do the job.
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post #3 of 9 Old 05-29-2016, 4:11 PM Thread Starter
 
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Re: Brakes won't bleed

So the bore in the master cylinder was very nice, no problem, the circlip holing the piston was a little rusty under the boot, but that was replaced, the kit is identical to the piston that came out, and I put the seals on the the same way. Now what I am finding strange is that the fluid in the reservoir does not really want to go down, if I take off the banjo bolt to the master cylinger and put my finger over the end, it pumps pressure nicely, but then sucks back a little which is what I would expect not to happen. I thought the master cylinder was more like a weak one way valve that would pump fluid out, then on the return would take fluid from the reservoir. The thing is I don't know how that is done, is there some kind of tiny hole that may have become blocked in some way?

I never did take the reservoir off the master cylinder itself, as I didn't expect it to be a problem, I just don't want to introduce any more problems.
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post #4 of 9 Old 05-29-2016, 4:44 PM
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Re: Brakes won't bleed

The "one way valve" action comes from the shape of the seals in the MC that only seal fully when there is pressure on the caliper side. If the pressure on the caliper side is lower than the pressure in the reservoir, it will allow fluid to leak past.

This means it's normal to feel a little suction on the return stroke, but also means that bleeding can be difficult when there is a lot of air within the brake lines and calipers. By far the easiest way to fix this is to reverse bleed the system using a suitable syringe to pump fluid into the caliper bleed nipples and back up into the reservoir. This both avoids the suction problem and pushes the air in the direction it prefers to go (i.e. upwards).
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post #5 of 9 Old 05-30-2016, 12:25 AM
 
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Re: Brakes won't bleed

Don't they have vacuum pumps in the UK, to keep the next step under 5 hours? Just saying.
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post #6 of 9 Old 05-30-2016, 2:36 AM Thread Starter
 
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Re: Brakes won't bleed

To be honest, this is the first time I have ever overhauled the master cylinder, and I didn't realise it would be problematic. The problem is that I don't have a bleeding kit nor do I have a syringe to reverse bleed. Looks like this is going to be a bigger problem that I first thought.

It's bank holiday Monday here in the UK, so I will be a little stuffed till tomorrow. oh well.
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post #7 of 9 Old 05-30-2016, 5:09 AM
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Re: Brakes won't bleed

Try your local supermarket for a basting syringe, that's what I use. They are designed for injecting marinades etc. into meat.
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post #8 of 9 Old 05-30-2016, 4:33 PM Thread Starter
 
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Re: Brakes won't bleed

BenelliBlade, spot on mate, I in the end I butchered a car bleeding kit, one of those plastic bottle with a hose that goes to a Cap that fits on a car master cylinder and a hose that goes to the tyre. I just cut the valve bit off and the cap for the master cylinder, then one end went onto the bleed nipple, and I pressurised the bottle with a track stand bicycle pump, worked a treat. Thanks for the explanation too, that helped immensely, I was about to strip the master cylinder down again thinking there was a problem.

Kind regards
Rob
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post #9 of 9 Old 05-30-2016, 4:36 PM
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Re: Brakes won't bleed

Nice one, glad you got it sorted
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