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Formula rx brake leak

arcadianarcadian Posts: 35
edited November 2013 in MTB workshop & tech
Only a couple of rides old, my rear Formula RX brake pretty much burst at the rear caliper, leaking brake fluid.

I'd given the brakes a couple of test squeezes as I remembered it felt a little spongy last time out, just heard a squish noise as the fluid leaked out.

I gave it a quick bleed, but the same thing happened.

I'm guessing the seal in the caliper/piston has gone wrong somehow, or maybe the piston. Ordered a replacement piston/seal kit. Only other thing I can think of is the bore ca might not have been tight enough.

Never had to change a piston or seal out on a brake, so was hoping someone might be able to offer a bit of advice on doing it?
Also, with the bore cap, it looks like it's need a special tool to tighten/loosen, as it's got 4 holes spaced apart - not got something like that, but would using the nose of a pair of needle-nose pliers in the holes to turn it work without damaging the cap?

Thanks :)


  • nicklousenicklouse Posts: 50,675 Lives Here
    arcadian wrote:
    I'm guessing the seal in the caliper/piston has gone wrong somehow, or maybe the piston. Ordered a replacement piston/seal kit. Only other thing I can think of is the bore ca might not have been tight enough.
    so where is it leaking from piston seals or bore seal?

    have you had a look?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Sorry, should have said, definitely leaking from the piston side. Watched it the second time after I bled the brake and gave the lever a squeeze to see if it would happen again.
    Just thought the bore cap on the other side might not have been fully tight, sort of guessing at why a pretty new brake would leak so soon.
  • Had a little bit of time today to look at the rear brake - what looks to have happened is the rubber seal has somehow caught in the piston and been pinched up to the surface (where the piston meets the backing plate of the pads).
    Because it's a one piece caliper, I can't get easy access to pull the piston out, but it seems pretty firmly stuck.

    Any ideas on how I might get it out? Also, the bore cap, my idea on how to remove that proved unfruitful, if I could get that off I would have access to the back of the stuck piston.

    Any help would be greatly appreciated, I'm a bit stumped with it :(
  • POAH wrote:
    buy a new one

    Haha, was actually tempted to buy a set of Shimano xt brakes and do away with the Formulas.

    Got an email from the US support for Formula, basically a guide on how they use compressed air to pop out stuck pitons. My track pump had an adaptor (sort of cone shaped one) which fit in the caliper, took a few goes but eventually got the stuck piston out.

    There had been a sort of imprefection, sliver of plastic that had somehow caught the rubber seal and dragged it in between the piston and caliper.

    New set went in after a bit of fiddling, brakes bled, new pads in and seem to be totally sorted. Should be able to get a ride out tomorrow morning, so will have a chance to rebuild confidence in them as my first impressions were they were really good brakes :)
  • 02gf7402gf74 Posts: 1,168
    Too late as you seem to have sorted it.

    One piece callipers will have a cap to close the hoke that is there to machine the piston bres. The caps are virtually impossible to remove without the tool - i know as i had to send my r1s away. This was to change the cap colour. Most pistons should be able to be remove like you have done or tying one with cable tie and pulling on the brake lever to pop the other one out.
  • I was thinking of getting an adjustable pin spanner to try and get the bore cap off.

    The US tech support email guy said they didn't remove the bore cap to remove pistons (he said, like you did, that it's just there for machining the caliper) but used air pressure. The instructions he gave me basically said to unscrew the banjo bolt, then blow the air through there, with bursts of around 120 psi enough to do it.

    Using fluid pressure hadn't worked as there wasn't enough pressure, the way the seal was trapped meant the fluid had a direct way out so not enough pressure could build up. With the track pump, the air pressure was enough to pop it out after a few pumps (I had cable tied the non-stuck piston pretty much like you say).

    When I'd searched online I found next to no info with similar stuff happening to Formula brakes, so hoping it's a pretty rare thing that went wrong with mine :)
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