Gradually softening brakes

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jchesshyre
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Gradually softening brakes

Post by jchesshyre »

I'm doing something wrong with my brake servicing but can't work out what. On both my bikes but especially with the VTR, my front brakes get gradually softer over time after bleeding. They never reach a dangerous stage but they just get spongy. Pumping the lever and/or cable tying it overnight does help, but then they get gradually softer again over a period of a few days.

- There are no fluid leaks
- I have HEL braided lines (using a fully separate line to each caliper)
- All banjo bolts are torqued to spec with new or re-annealed washers
- Brake fluid is DOT 4 from a sealed container

After bleeding I'm able to get a firm lever, but as I say this deteriorates gradually so somehow air is getting in. What could be the cause? Can bleed nipples leak a bit so as to allow air in but not fluid out?
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MacV2
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Re: Gradually softening brakes

Post by MacV2 »

I always use a bit of PTFE tape on me nipples... 8O

Also you may have some air trapped in the M/C so if you try bleeding that it may help...

Worst case is the M/C is in need off a new seal/ refurb...

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popkat
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Re: Gradually softening brakes

Post by popkat »

Are you getting air come out when you first bleed them ?
It's not normally possible to get air into the system, you could remove bleed nipples, clean up threads and put some ptfe tape on them, be very careful not to let any tape into the hole though, leave it back a bit from the inner end, this will eliminate any possible cause there.

I have never got a really solid lever on a VTR, they are always a little spongy to pull in but work well on the road, i do find i need to put fresh fluid in every year to keep them good.

another thing is your bleeding method, I use a long bleed pipe going in to a pot, (in my case an old mitivac pot) i them raise the pot up as high as possible, open bleed nipple and pump, when I see no more air I close the nipple, give it 5 seconds then pump the lever, it should only take 2 or 3 pumps to get a good lever, then I hold in the lever and crack the nipple open then closed, let lever out and leave 5 secs then pump, I do this a couple of times and it's done. always start with the caliper furthest away from the master cylinder (near side)



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Wicky
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Re: Gradually softening brakes

Post by Wicky »

Can bleed nipples leak a bit so as to allow air in but not fluid out?
Check that bleed nipple hasn't been overtightened at some point and caused a microcrack around thread that could allow tiny fizz of air bubbles to enter system - as Mac suggests use PFTE tape.

For bleeding I use a large catheter syringe and draw fluid down keeping reservoir topped up so air can't enter from top. Takes 5 minutes to bleed both sides.
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jchesshyre
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Re: Gradually softening brakes

Post by jchesshyre »

Yeah I'm confident that I'm bleeding them OK – it takes a while but I get a firm lever eventually. I bleed from all banjo joints as well as from the nipples, and plenty of bubbles come out.

I overhaul the m.c. according to the Honda schedule (24,000 miles). This is due in 2,000 miles but the issue has always been there so I don't *think* it's this.

I suspect that I over-tighten the bleed nipples. I use a torque wrench for most things but not these as mine doesn't go down to 6 Nm! I might get new nipples and use some PTFE tape and see if that sorts it. Even the Honda manual says 'If air is entering the bleeder from around the bleed valve threads, seal the threads with teflon tape' so I guess it's quite a common issue.
jchesshyre
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Re: Gradually softening brakes

Post by jchesshyre »

Right, I've replaced the caliper bleed nipples and applied Teflon tape to the threads. I've bled the system again with fresh fluid, from the nipples and from both banjo bolts. No visible bubbles emerged anywhere.

However, the f*cking issue is still happening. I can get a reasonably firm lever (although still with more travel than I'd like) by pumping it, but within a few miles of riding it goes soft again (this is starting to sound weird...). And this is with the lever adjusted as far out as it can go - if I screwed it all the way in it would probably reach the handlebar before any real braking occurred!

What the hell can I check next? I've never had this problem with my CB, but that is a single-caliper setup and is a sliding caliper (not that I can see those details making a difference).
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MacV2
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Re: Gradually softening brakes

Post by MacV2 »

time to check the M/C more closely... Either a rebuild kit or try another M/C...
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jchesshyre
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Re: Gradually softening brakes

Post by jchesshyre »

MacV2 wrote: Thu Nov 15, 2018 3:17 pm time to check the M/C more closely... Either a rebuild kit or try another M/C...
Well it is due for overhaul anyway, as I last did it in July 2017, which seems to be 25,000 miles ago :lol: It has been like this the whole time, although has possibly been getting very gradually worse over time. I've checked my emails and did use an OEM rebuild kit.

*edit* oo and David Silvers have an OEM one for £21.95 - bit better than the £50 or so that is the normal price. I nearly got a £34 TourMax one!

What should I look for when inspecting the m/c and how/where do they tend to admit air if faulty?
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MacV2
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Re: Gradually softening brakes

Post by MacV2 »

jchesshyre wrote: Thu Nov 15, 2018 4:39 pm
MacV2 wrote: Thu Nov 15, 2018 3:17 pm time to check the M/C more closely... Either a rebuild kit or try another M/C...
Well it is due for overhaul anyway, as I last did it in July 2017, which seems to be 25,000 miles ago :lol: It has been like this the whole time, although has possibly been getting very gradually worse over time. I've checked my emails and did use an OEM rebuild kit.

*edit* oo and David Silvers have an OEM one for £21.95 - bit better than the £50 or so that is the normal price. I nearly got a £34 TourMax one!

What should I look for when inspecting the m/c and how/where do they tend to admit air if faulty?
:Shrug:
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Wicky
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Re: Gradually softening brakes

Post by Wicky »

If you can take the complete system off i.e. master cylinder, hoses and brake callipers and rig them up indoors somewhere warm & dry. Bleed em nice and tight (put some wood or something between pads) and observe if fluid is dribbling out slowly over time anywhere - say with sprinkling talc over likely culprits.
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