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Suzuki GSX250F Across - Front Master Cylinder

Discussion in 'Technical and Troubleshooting Torque' started by pokolbinguy, Jan 10, 2015.

  1. Hi All,

    I am having a fantastic time (read as "pain in the butt) trying to get my front brake system working again after it locking up on me (for a second time).

    I have pulled the front caliper apart and given it a good clean up. The caliper seals were replaced about 12 months ago (only ridden twice in that time) after I had the brakes lock up on me the first time.

    This time around I suspected the master cylinder to be the culprit so pulled it apart to find some gunk in there like the brake fluid had gone "off", pulled it apart and replaced the piston & cup set.

    Pushed new fluid through the lines, all seemed good there.

    Now I can't seem to get the M/C to prime properly and force liquid down the lines. I can push liquid from the caliper all the way back up to the banjo at the M/C no issues and also into the M/C through the return port (while it does take a bit of a push on the syringe to get it to go back into the M/C.

    I have a vacuum bleeder kit and thought that would sort the issue but I can not for the life of me get the system to work properly and get pressure on the lever.

    It is sending me around the twist.

    Any tips?

    My next thought is to try and replace the M/C itself and probably the brake line.

    Anyone got a good condition / functioning GSX250F front master cylinder laying around?

  2. i remember when replacing the fluid in my across it seemed to take forever to get the fluid into the lines from the mc. i just kept at it (not sure if i left the valve open or closed on the caliper) and it ended up going through.
    there is a guy in brissy that may have 1. go to 2fiftycc.com and search kiffsta. to bloke, i got a set of forks from him that were in real good shape. otherwise just ring a few wreckers.
  3. Yep I have tried it for a bloody long time and different methods and seem like I am getting no-where. It is very frustrating. Any idea how long you think it took you (constant bleeding time?) ?

    Small world I bought a spare front caliper of him 2 years ago. Sent him a message a couple of hours ago to see if he has the part.
  4. i think it took me something like 10-15mins of constant bleeding. i must have done something wrong for it to take that long though. but did work in the end.

    did he sell the caliper your using now?
  5. Ok, I think I have tried longer than that. It did cross my mind "am I doing this for long enough"

    I didn't end up using it. In the mean time while I waited for it to arrive I pulled mine apart and cleaned it all up and could not find anything wrong with it so put it back on.

    I got the one I got from Kiffsta out today, it was pretty damn dirty, pistons were stuck in. They took a bit of effort to get out. I will give it a clean up and keep as a spare.
  6. hey just a thought. did you undo the bolts holding the hose on? there are holes in it to allow the fluid to pass through into the hose upload_2015-1-10_22-54-31.
    as for the locking up it might be better to wait for the more experienced to answer. good luck mate
  7. I've got one off a sv650, don't know if its the same. What numbers are written on yours
  8. These are the banjo bolts. I pulled these out and cleaned them out. All good.
    • Like Like x 1
  9. The M/C says "Nissin 1/2"

    Looks just like this;

    Attached Files:

    • mc.
      File size:
      296.3 KB
  10. So I

    1 - pulled the master cylinder apart again, gave it another clean, put a pin through the return hole, cleaned up piston/cup set and put back together. MC is pushing liquid out as it should

    2- drained caliper of old (actually new but ditched it any) liquid

    3 - took brake hose off, no damage or cracks from what I can see.

    4 -connected brake line to caliper and pushed new liquid through (all at ground level (brake line off bike) and into a clean jar. Clean fluid came through, no gunk/cloudy liquid and stream was constant.

    5 - connected system back together on bike and can get M/C to push on pistons at caliper and liquid to flow out bleed nipple.

    6 - have pushed through or pulled through (with vacuum pump) easily 10 x MC reservoir full of liquid (didn't let run dry so no air entering at MC) but the pistons will not go hard on the rotor

    There is deffinatley pressure on the pads, if I hold the lever in and push the bike you can feel the pads rubbing the rotor. When you let go of the lever the pistons at the caliper retract which I assume is the problem.

    7 - after realising the pistons are moving back upon release of the lever I took the banjo off the MC to make sure it was bled properly. pull lever in, liquid comes out, finger over hole, let lever go to stop air being sucked in. All seems ok.

    8 - connected back up, can again push liquid from MC down the lines to caliper and out bleed nipple. Pistons at caliper still move out when lever pulled in, then when lever let out they move back and therefore won't go "hard".

    What am I missing????
  11. Sounds like a large bubble of air in the calliper. The MC pumps, You can move the calliper piston with the brake lever (compressed air can apply a force up to a point, but your lever will never get "hard" if it's compressing air. It works, but not satisfactorily. Air in the system is the most common explanation for soft lever travel. Another, less common, is fluid leak. In an old bike, there might be wear in the MS, or bad seals. You've eliminated some of these as possibilities, except for possibly wear in the bore of the master cylinder. The lock is a mystery. You didn't mention whether there was any sponginess in the lever action. No air = rock hard lever, in theory. Any sponginess is then due to your brake lines swelling due to being pressurised. Most do, a little.

    I'll try this on for an explanation of the "lock". Air expands a lot as it is heated. The brake pad/disk interface is in direct contact with the brake calliper piston - there is a big heating effect, nearly instantaneously when braking; this heat is transmitted through the pads to the piston which touches the fluid and maybe air - a large expansion of air keeps it on until you work the lever a bit, and it cools down somewhat. There is no "spill" of brake fluid apparent in spite of the pressure in the system because it's a sealed system. Alloy parts transmit heat pretty well and it all settles down fairly soon.

    I'd be looking at the bottom end of the system. There are some callipers which are notoriously hard to get the air out of due to the angle they are mounted, or design. Also, when bleeding, if the nipple is loosened too much, the threads can leak air back in when you release the lever, in spite of one's best efforts at keeping this from happening by immersing the tube in a jar of brake fluid. It also helps to loosen the nipple only enough to let fluid out, while applying a slow, smooth stroke to the lever, then tighten it before you stop the lever stroke. It's tedious, but absolutely prevents any air entering through the threads when you release the lever.

    Top-down bleeding, while it works, is always pushing fresh brake fluid down against the bubbles which are "trying" to rise. There is a certain amount of faith that the rate of pumping from the top will carry enough fluid fast enough to carry all the little bubbles trying to rise to the top down and out the bleed nipple faster than they can rise and that while this is happening, fluid will trickle through the channels in the calliper to the piston area against air exiting. This doesn't always work so well, particularly in motorcycle systems which don't pump much fluid with each stroke, and those little channels in the calliper are not large.

    Some people resort to "Bottom Up" bleeding. Fitting a large syringe (like you might buy at Supercheap) of brake fluid with a bit of hose to the bleed nipple. and feeding the system fluid from the bottom. This way, bubbles now rise with the flow into the reservoir. Don't forget to remove as much fluid from the reservoir as you add - a second cheaper, smaller syringe is helpful.

    If this still hasn't got air out of the calliper:

    The toughest of systems can be rendered free of any air by tilting the bike or demounting and inverting the calliper so that you are adding fluid from the very lowest part of the system allowing air bubbles to rise, in fact pumping them to the top.

    Honestly, it shouldn't come to this - this is a mongrel procedure for only the most troublesome of systems.

    If you demount your calliper(s) ensure that the bolts are done up to the correct torque when you remount them.

    This may or not be helpful.
    • Like Like x 1
    • Agree Agree x 1
  12. Thanks for your post. Its helpful....to a point....where I feel I have tried all of these options and still not gotten anywhere.

    The air in the caliper scenario seems logical but I am sure I had excluded that by pushing liquid from the caliper up the system as you suggest with your explanation of "bottom up" method.

    I really feel like I am going in circles. I have spend 6 hours going around around and around today plus a number of hours over other days trying the same things.

    Either something is not right and I haven't found it or I am completely useless.

    When you mention "wear to the bore of the master cylinder" - what should i be looking for? I can't seem to see any scuffing / corrosion / pitting or anything similar.
  13. I think it is unlikely, but in OLD bikes, the master cylinder can actually wear out. Your bike is not that old. It pumps. Worn? to confirm, block the output with a plain bolt so it is sealed. Try it then. One would expect your lever to be rock hard - there is nowhere for the fluid to go. A worn one or one with a bad seal will squeeze down, slowly, as fluid escapes past the piston and/or seal. It may also leak. Don't get TOO enthusiastic with the squeeze - it should resemble what you do when braking.

    If it does seems OK and you can pump fluid down-up style into the reservoir from the bottom with the lever released, but not if the lever is pulled (actually pumping fluid down) then It allows fluid to return when the lever is released and pumps fluid and holds pressure when the lever is applied, then your problem is clearly elsewhere.
  14. You say the brake has locked up on you for a second time? What was happening when that happened? Do you mean locked up while riding or locked up (seized) as in lack of use (sitting) Is the correct caliper fitted, as you seem to have followed the correct procedure to bleed brakes. Yes, the pistons will move back a little after letting go of the lever. Are there any caliper slides that need attention, are they seized? Just throwing ideas around here, hoping to help.
  15. Was riding the bike along and the front brake slowly locked up. From this I can only assume there was air in the system that heated up and expanded and therefore locked the brakes up. This was the same thing that happened the previous time. Bike was only ridden for a short time, say 10-15 mins before it happened.

    Same caliper that was on the bike when I got it that worked perfectly for years.

    Rightio, good to know. I am still stumped to why I can't get it hard though....god it sounds like I should be going to the doc and get a prescription to solve this issue!

    Seems to move back and forth freely. Has been pulled apart and greased
  16. Water in the system? - water will boil at relatively low temperatures. I hadn't thought about that. In a closed system, as your front brake is, there'd be nowhere for the pressure to go apart from pushing on the pistons. Brake fluid readily absorbs atmospheric moisture - the reason they always advise to use new brake fluid only, from an unopened container. It is normal for brake parts to get quite hot in use, even if the brakes are not in use, due to a certain small amount of "drag" of the pad on the disk. Avoid using the brake and coast in to a stop and touch your disk - you will see what I mean. Rear brake reservoirs are usually vented, front ones are often not.

    Just another thought.
  17. Could be what caused it (water).

    Down-side is I still don't know how to fix it and get this thing working properly.
  18. Just a wild thought. Is the bleed nipple on the caliper at or near the top when it's sitting in position? If not, you'll have little chance of removing any air using normal bleeding methods. Don't laugh, Ive seen this before when calipers have been refitted, the port for the bleed nipple is the same thread as the one for the brake hose, presumably to make the caliper a sort of universal fit, and the fluid line got fitted to the upper port.
  19. Yep is towards the top
  20. Given I seem to be going around in circles I think its time to resort to seeing a professional.

    Any tips on an "old school" bike mechanic in the Cessnock area NSW?