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Front brakes locked up, but I wasn't braking...

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by gsxrdan, Jun 12, 2007.

  1. So I picked up an '86 VFR750 with 52k's on the clock - reasonable condition... drove 18 hrs there & back in the ute to get it (glad I didn't try to ride it back...) got the thing off the ute next day & rode it to servo to fill 'er up for a cruise. Didn't quite get to the servo though, only got about 1.5 km's when, while cruising at 50 with traffic, accelerating up and around a corner (not applying front brakes) I all of a sudden came face to tarmac with the realisation that whilst my new bike was sliding toward opposing traffic, i'd just been highsided... I was very confused? so after the obligatory "Oh, sh*t, my bike!!", Iv'e run/hobbled over to it and picked it up, couldn't get it out of gear of course, so the back would have to be slid off the road, but my growing confusion, about what the hell had just caused the seat under my arse to be replaced by abrasive bitumen, was intensified when the front wheel wouldn't roll. The front brakes were seized on, and the brake lever was as solid as a rock - couldn't pull it in at all, in fact the lever was flexing slightly, but couldnt move the plunger in the master cylinder... 1 disk was very hot... it was then i realised that there had been some drag on the bike, noticed while clutch pulled in and coasting, but i had thought nothing of it at the time, (hindsight is a marvelous tool for chastising one's self) After an hour, the lever was still solid; after 3 hours, it was back to normal; after three days I can now move around a bit - but im suspicious about the crunching in my knee cap... Anywho, what the? I've considered grit or corrosion in the calliper or warped disk/s causing the system to drag & overheat, but wouldn't that reduce the pressure in the hyraulic system? What could increase the pressure to the point the brakes apply themselves to wheel lock-up point? Pertinent facts: I dont think it had been ridden for a while; the brake fluid "appears to be" in good condition; bike rolls with minimal resistance now; there is no obvious brake pad rubbing noise; brakes work appartently normally now (but i'm no guinnea pig, me!); I didn't check the air pressure while it was on the ute, was heading to the servo for that - stupid man... front tyre pressure is low - guessing 20'ish psi; forks very soft - but that's compared to the Marzochi's on my other VFR, maybe these are meant to be soft? So what the hell went wrong???

  2. At I guess I'd say it's possible that the brake may have been dragging a little - this could have caused the fluid to expand and lock the brake on (especially if the fluid level was too high). Pure speculation here but possible that the owner may have known about the problem, and that it was losing brake fluid, and decided to "put a little extra in" before selling so it'd take a while before you noticed the problem.
    Edit: Or they just plain stuffed up and put in too much fluid.
  3. Thats scary, hope ur alright

    Brake fluid has a pretty damn high boiling point. Like 300 degrees. It would have pretty serious brake drag to cause it to overheat.
    Maybe someone has put water in the brake fluid, it would overheat then.
    Or wrong DOT fluid.
  4. Car related but this seems to sum up your problem, and its possible cause, fairly well:
  5. Yeah it needs a kit putting through the caliper. The pad dragged and everything heated up. I had it happen on a rear caliper once.

    Though I am surprised it come on quickly enough to cause a sudden accident. Maybe you applied the brake and it jammed on.
  6. I had this exact problem on an old GPZ750, I started at the calipers and just wasted my time. Had to rebuild the master cyl.
    In my case all the cups had swelled in the master cyl and would not release the pressure as they should, so every time I used brake a little more pressure would stay in the lines until virtual 'lockup', if left o/night all would be back to normal.

    I was lucky and it didn't throw me, I realized I was wringing the throttle out and going slower. It didn't register in the brain though when I looked over the bike and noticed ..... er that front disc looks hot and grabbed to 'feel' (burnt 3 fingers on a virtual hotplate :oops:

    cheers :)
  7. Thanks all - theres more to the story...

    Thanks for your thoughts, I think Bob may have hit it on the head there with the seals, but wait theres more... Now Ive been over the bike with a fine tooth comb, have found that 'someone' has replaced the brake reservour lid with an ill-fitting alloy plate, I can see fluid weeping under the squished seal; also theres a crack in the line from the master cylinder where fluid is seeping... methinks therefore water has been absorbed into the system, would this have affected the seals? Would water contamination cause hydraulic lock? As an aside, the fluid level is within the specified marks. Will hit it with a brake kit then, and let someone else test ride it... :p !
  8. Re: Thanks all - theres more to the story...

    Absolutely. Water will lower the boiling point of the brake fluid and cause it to expand a lot more under heat.
  9. pull apart the front calipers to see what you need before you order a kit. Kits only come with seals and you might need caliper pistons aswell if they are pitted, theres no point doing the seals and leaving in stuffed pistons that will need replacing a few months down the track.
    its probably also a good idea to get a good second hand master cylinder from a wrecker if the resevior has a crack and a bodgy cap. rather than putting a kit in the cracked one.
    while your at it, its not a bad idea to check the brake lines for cracks and get the front wheel off the ground, and with the calipers off spin the wheel and check that the discs arent bent/warped.
  10. Sounds very nasty to me, lucky ecsape.
    I also agree with putting new seals in everything, including master cyl. I would also get a new m/cyl cover and gasket.
    When you rebuild the calipers, be sure and thoroughly clean the guide pins if it is a floating caliper, when I got my GTR, all three calipers on teh bike were partially frozen in the slides, causing immense brke drag (I couldn't push teh bike).
    Most service manuals I've seen recommend rebuilding calipers and m/cyls every two years, and whilsts that is a bit overkill, it shows how important it is. At the very least, everything I own gets fresh brake fluid annually, a good compromise in my opinion.
    Whoever did that dodgey cover to the master cyl should be shot. I always spend a couple of days when I get new wheeled transport going over looking for little things like this, as you've found out, they can turn nasty quickly.

    Regards, Andrew.
  11. Your calipers gummed up with crappola I would say. Your disk rotor will never run 100% true and the caliper pistons were gummed up with crap probably, so they cant release there pressure being applied on them by the master cyliner.
    So basically are being applied permanently. Couple that with the ever so slight wobble in the front rotor at speed,everything heating up expands and warps and whalah,hello Mr Bitchumen :? Rebuild braking system time me thinks. Specially seeing as you dont know history of the bike,
    coz if 1 caliper is gone, tother is usually not far behind.

    Just been through this on my Diversion.Replaced the master cylinder but just revealed next weakest link being calipers, that had old seals and crystallised brake fluid and basically just clogged with crap.Pistons luckily were salvagable and not too pitted.
    I had an old GS1100 rear brake almost seize and rear caliper catch on fire at 100kmh :?

    It aint a hard job if your ok mechanically.All up for master cylinder kit and 2 caliper seal kits,cost me 220 bananas and about 4-5 hours labour(3 of which is cleaning) for essentially a rebuilt front braking system and truckloads of peace of mind after several close calls :shock: due to inadeqate froont brakes



    This site gives you a really good view of whats needs for your master overhaul.

  12. crappy brakes anyway

    To be honest, the old nissin 4 pot callipers on these bikes are pretty crappy anyway, they kept using them at least till 91, probably beyond... but theyre spongy even wityh braided lines, and i think the discs are only about 250mm, don't know, but they're not much chop... my other vfr has rgv discs(320-ish mm - & lighter rims to counteract the weight of the discs) with tokiko 6 pot calipers and braided lines, same brakes as my 96 gsxr750, they haul ass, I think this old girl will just serve as parts to get my project registered, then be sent to the knackers via ebay... beaut engine if anyone needs a buggy motor... Cheers for all the input, i certainly walked away a little wiser for the pain... Dan.