Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Lobo on August 07, 2013, 10:00:54 PM
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The front brake on my 400F is spongy versus my K2, where you can barely pull the lever before the pressure gets too much for further movement.
The 400 has new hoses, and has been bled, bled, bled... no bubbles / cloudiness... nothing but clear fluid. And stiil, (with a lot of effort, granted) I can pull the lever back to the handlebar. MOT fail?
Would the master cylinder benefit a rebuild with new parts / seals to cure this? I only ask because I have my doubts ... due the pressure does not 'leak away' as you hold the brake lever on. (and if the consensus is this... the DSS kit ok / recommended?)
Answers, suggestions please? I can't believe the difference in my two old & very similar systems; I've got the two extremes.....
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have you tried dead legging the leaver (tying it back to the bars over night and loosen cap on resovior
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http://www.sohc.co.uk/index.php?topic=4812.msg22283#msg22283
I refurbished mine using the genuine kit from DSS
Does the master piston move freely or is all your effort going into not moving it much (is it seized?)
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Thanks guys.
Busa, nope, not tried that.... 'what' is that doing? Return to work (abroad) today, but will try when I'm back in a couple of weeks.
Tom - thought you might have it, but then realised it 'pumps' (moves freely) fine when bleeding, with good fluid flow, so I don't think that's the cause.
Keep the thoughts coming!
Simon
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have you tried dead legging the leaver (tying it back to the bars over night and loosen cap on resovior
'what' is that doing?
It lets any micro-bubbles rise to the MC. I do it everytime I bleed my brakes and it does improve the pressure.
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Ta Lester, as I mentioned, will give this a try.
Does seem to be 'opposite' to what deep sea divers experience, ie bubbles forming (in their bloodstream) as the pressure reduces (the dreaded bends)... but who cares as long as it works!
Cheers,
Simon.
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Thanks lester. I do this even with cars its just make any air seep back up the pipe.
just a thought have you tried bleeding it from the front brake light switch aswell. or from the banjo at the master cylinder this will also get an air lock in it and not go down the brake pipe..
or reverse bleed it open caliper bleed nipple use a syringe and small bleed pipe, drain all fluid out put new in from caliper end but do not pull brake lever. keep ya eye on fluid level in master cylinder so as not to over flow. squeeze fluid in slowly once bubbles stop coming through the master. leave syringe and pipe attached until you close the nipple.
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Thanks busa, like the idea of reverse bleeding, though have little intention of undoing mid-joints due the mess & awfulness of brake fluid!
You guys given me a couple of new possibilities - cheers!
Simon
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I had a similar problem with my old GSXR.
Eventually, I found that the seals in the calipers were not allowing the pistons to slide freely. The pistons were moving to put pressure on the pads but the seals were flexing as the pistons moved and when the lever was released the seals pulled the pistons back. It took ages for me to find this but you could see the pads moving back when you looked closely. On strip down of the calipers, I found a resin-like build up - mainly around the seals and I the seal grooves.
Once cleaned out and reassembled with new seals I had a solid feel at the lever after about a minutes bleeding, whereas before I was bleeding the brakes for ages for no improvement.
If you haven't stripped your caliper for a while this may be worth checking if nothing else is working 👍
Dan
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Dan, the seals are supposed to pull the pistons back, in fact that is all that gives pad/disc clearance
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Hi Mate,
I posted about spongy levers a while ago and have pasted it below. Hope it helps. Ps Make sure that the little hole in the reservoir is not blocked. You can carefully poke it with a needle, (I did say carefully) to clear it out. the fluid must have a method of return and this little hole is it.
cheers
john
Hot tip for bleeding brakes after refitting pistons and seals!
Half fill reservoir and GENTLY pump fluid into system topping up as you go until it is obvious no more fluid required.
IMPORTANT Do not force the lever at any time as this will compress the air into the fluid, yes brake fluid will suck in air and it will become elastic.
Leave overnight. The weight of the fluid will want to go down and the air will want to come up. In the morning the air will be at the top of the system, and by GENTLY moving the brake lever in and out WITHOUT APPLYING FORCE you will see lots of bubbles rising into the reservoir. Do this for several minutes until the air bubbles stop rising. Leave for 15 minutes and then gently move the lever again to see if there is any more air. Repeat if necessary until no more bubbles.
The important thing about this process is not to apply pressure on the lever until the system has no air in it or you will compress the air into the fluid and this will give you a spongy lever.
Also you should avoid leaving the cap off the reservoir for long periods as air will ingress into the fluid merely under atmospheric pressure.
This method requires a little patience but beats the s--t out of spilling fluid over your nice clean bike!
Oh! One more thing, always use new fluid, if it has been in a can that has been opened for some time it is useless.
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Bryan - agreed, but not to the point that you can't get a solid feel at the lever. And there shouldn't be crap built up around them from road contaminants, salt, etc.
After a caliper overhaul my similar problem with brakes was fixed and its an easy and inexpensive task if nothing else is working.
Dan
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The seals on my bike had so much crap built up around them that they were basically glued to the pistons and not allowing the pistons to move through them to compensate for pad wear.
You could bleed the brakes all day long (pointlessly 😤) but still have a spongy feeling at the lever because more travel was needed to get the pads against the disc.
This will probably turn out to not be your problem Lobo haha
Dan
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Hi gents: thanks.
The bike has just undergone a major rebuild, and whilst the m/c wasn't touched, the caliper was cleaned / repainted, as was the intermediate pressure switch junction, and all hoses / pipes renewed OEM.
It has been bled...and bloody bled... carefully, by the book, with nothing but fresh DOT 4. The posting above above is in opposition to previous ones... ie 'no pressure into the fluid.. leave overnight' versus maintain pressure overnight.... but all to no avail. (surely the 'no pressure / leave overnight' method is what we're doing 365 nights over the year? - ie the brake should tighten up with time?)
I can only see that I'm down to a master cylinder overhaul now; and am quite happy to spend that cash as everything else about the bike just about perfect. Will report back...!
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Lobo
I did what you did (I assume you used a new seal in the caliper) plus (as stated earlier) the m/c overhaul kit. I used Earls hoses but I don't reckon that makes any difference. Will post pic later of my bleed technique. I'm pleased with the overall result (using EBC pads). Firm and effective (using four fingers as others have said).
Aside: took it for MOT last Sat. Apart from the general furore surrounding her the thing the test guy remarked on was the thickness of the disk!
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I had problems bleeding my brake last year, just could not get a good firm lever, in the end I left it in disgust, went back a few days later and had a good brake! I think the air bubbles just rise to the top over time, too much bleeding keeps agitating the fluid and adds more air.
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Brake fluid sucks in air, that's why you shouldn't leave the cap off for any length of time. If there is air in the system and you compress the system the air will ingress into the fluid instead of rising to the top causing the fluid to flex before pushing the piston.
if you can't bleed the air out it is best to let it rise to the top of the system and filter it out by wiggling the lever gently. You will be surprised how effective this method is.
It was the only way I could get the air out of my VF750FD clutch.