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B***ard Brakes!

Discussion in 'Technical Help' started by jimmer, Oct 12, 2021.

  1. First off this is non-Ducati but it's all hydraulics so how hard can it be. In summary I have a 1980 Suzuki GSX750 (with an old GS750 engine, no idea why) and despite my best efforts I cannot get the front brakes bled up, the lever just goes back to the bar. Any tips are welcome as this is driving me up the wall.

    The long version. I bought this bike as a bit of a rolling project back in February, all pretty good but the front brakes were rubbish and just needed a few jobs finishing off. Rode the bike home and the lever was coming back to the bar and there was very little stopping power, thankfully the rear brake works brilliantly so no drama. Tried bleeding the brakes when I got home with no improvement at all but while I was doing it I thought the return action on the brake lever was pretty much non-existent so decided to rebuild the MC. While I was at it I thought the hoses looked pretty guff and might as well rebuild the callipers while I'm at it. So rebuilt MC, callipers and new HEL hoses and gave it all another go...yup, you guessed it, still back to the bar :mad:

    I've tried conventional bleeding, pushing fluid from the callipers with a syringe, vacuum bleed and just about every other variation or technique I can find on Youtube and still no joy. The callipers are moving freely and the fluid that I get from both nipples is clean, clear and free of bubbles so I can't get my head around what has gone wrong. There does seem to be slightly less pressure from the left hand calliper when I open the nipple so not sure if that's a sign of something and I noticed a fair bit of resistance when reverse bleeding.

    The brake set-up is a basic 14mm Nissen master cyclinder (I think from a newer, circa 1990 GSX600F), twin braided lines and two callipers which are basic single piston sliding callipers, pistons are 38mm.

    So, answers on a postcard, nothing is too simple or too stupid for me to try before giving up and sending it to someone else to do. I used to think that I was reasonably handy until this came along :sob:
     
  2. Basically the whole front brake system is an unknown quantity. Assuming that there is no air in the system,
    Are you sure the seals are fitted the correct way round in the master cylinder, (don't just assume whoever fitted them before, fitted them correctly) ?
    If there is no air in the system then for the master cylinder lever to move to the handle bars without pressurising the line/s then fluid must be bypassing the piston seal/s.
    Try another, known, good master cylinder. Any working one will do, it doesn't matter what the piston diameter is, within reason. If it produces any sort of pressure then you know the issue is with your original master cylinder.
    If the replacement, known master cylinder does not produce any sort of pressure then there is air in the system.
    Ensure when you are trying to bleed the system that you orientate the bike such that the fluid path is always uphill. This will reduce the likelyhood of air getting trapped in places where the fluid is trying to go uphill, then downhill, then back uphill, if that makes sense.
    Sometimes putting the bike on it's sidestand with the steering fully to the left will help ensure this.
     
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  3. Cheers @CAT3

    So it's all been rebuilt by me, new MC seals, new hoses and new calliper seals. Don't have a spare MC kicking about but I've tested this one off the bike and it definitely builds up pressure, also bled from the MC banjo and the fluid came out at a fair rate of knots.

    Considering taking everything to bits again to triple check my work and then refill the system before committing to garage time.
     
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  4. Similar problem. Eventually removed both callipers. Placed old pads between new pads on each calliper. Hung both callipers above master level, left over night and bled whilst hanging. Big pop which cleared the air lock sitting in the lines or calliper.
    I had replaced and rebuilt the system before the above, twin lines from the master cylinder can cause an air lock. Never had a problem with 3 piece and a splitter set up. Hope that helps. Only other thing I can think of is duff bore or master seal.
     
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  5. Agree
    Get the calipers higher than the master cylinder.... That'll do it.
    I've had this in the past too.

    Just fill the master then open a bleed nipple with tube and usual jar etc connected then pump the lever .... Don't worry about opening and closing bleed nipple until you get some fluid moving. When it starts to build a little pressure then bleed as you normally do but continue with elevated calipers .....may as well do. If you had bleed nipples up on the master cylinder you probably wouldn't have to do this. This is only happening because the system is completely empty.....or you have assembled something wrong ....

    Cheers
     
    #5 Gerr, Oct 13, 2021
    Last edited: Oct 13, 2021
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  6. Have you tried bleeding from the caliper end with a vacuum device (e.g. Mityvac)? Works a treat for me on a clutch circuit (OK, I know it's not brakes...).
     
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  7. First thing for me would be to park bike with bars full lock to the left, pull brake lever in and cable tie lever to grip. Leave like this for 24 hours and it helps air work it’s way upwards.

    Next step is to remove cable tie and pump brakes around 6 times, then hold lever in and crack the banjo at the MC. Obviously make sure you have protection on any paintwork. Repeat this process around 6 times and then move down to bleeding calipers. I’ve always bled from the top and worked my way down, never ever failed with this approach even on the most stubborn of systems !

    good luck
     
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  8. Cheers for all the pointers guys, very much appreciated.
     
  9. Yes, tried a vacuum pump and pulled through a good load of fluid but still no improvement.
     
  10. Any luck with the brakes mate ?
     
  11. Not been back to them, had a weekend away and busy with work. Hope to get an evening this week to have another go at them.

    Fear not there will be an update, I hate an unresolved forum thread!
     
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  12. Some progress made.

    Bleeding from the MC banjo made no difference at all so took the callipers off and tied them up high, left them for a bit and then gave bleeding them a go. Decent amount of air came out but then one of the G-clamps I was using to resist the pistons shattered so put the callipers back on the bike to continue bleeding. Was back to square one with the callipers on the bike, no pressure at all so will find some alternative packers and revert to bleeding with the callipers hung high.
     
  13. Wow , unlucky… never had a stubborn one I can’t cure. Sounds like this one is a right pig. Good luck !
     
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  14. Hang them high. Packing can be two pieces of flat steel. I used old pads, just happened to be available. Leave overnight or longer, bleed nipples upper most. If it is a simple air lock it will work…..eventually
     
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  15. Still at it, still no joy
     
  16. Master cylinder diaphragm, master cylinder bore ok? Any fluid above the diaphragm? Reservoir to master cylinder hole blocked? Thinking out loud now. Free and sufficient movement on calliper pistons? Got to be something simple, somewhere! At a loss at this distance to come up with a definitive.
     
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  17. Everything is looking ok but I’ve noticed that the calliper pistons move back and forth across the same range of movement as if they’re being sucked back when the lever is released. If you pump the lever they don’t get progressively further out of the calliper…

    I feel like a strip, clean and start again is on the cards.
     
  18. Blocked reservoir to master cylinder, poor seals in bore. Mc too low rating?
    You will fix it!
     
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  19. Tbh by now I’d have just taken this as an opportunity to upgrade and replaced the lot!!
     
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  20. I think you very may well have a problem with the way in which fluid is passing between the reservoir and the master cylinder bore, controlled by the piston seal. The pistons in a caliper should retract just a little when the lever is released, but then they stop moving (due to friction between the pistons and the seals in the caliper, which are quite stiff), and the master cylinder piston continues to retract, but with fluid coming back into the circuit from the reservoir via the port, which should be uncovered after the lever has come back far enough. If your pistons go all the way back into the caliper again as the handle comes out, then it's "pulling them back" because the circuit is still more or less sealed from the reservoir, so each time you pull the lever, it comes all the way in because it has so far to move the caliper pistons out each time. Bearing in mind the frightful risks if a master cylinder should fail completely, I agree entirely that a very careful strip and check is necessary, or perhaps ideally a replacement master. T

    The port can't be completely blocked, or you'd get different symptoms, but there might be a problem with the seal on the piston, perhaps wrong size, distorted in some way, or whatever. The failure mode in this area can be difficult to make sense of. I once had a 1970s Mini 850, which had a hydraulic clutch mechanism, and I noticed that it no longer had any clutch pedal free play. It seemed to be possible for fluid to go from reservoir into the circuit (hence the clutch pedal still worked) but it wouldn't bleed back into the reservoir, as if the port had become a one-way valve. I didn't bother to rebuild it because it was so cheap to put a replacement cylinder on from a scrap car, and that fixed it.
     
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