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matgriff

290 front caliper rebuild

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Hi, I recently purchased a nice 2009 290 to start getting back into off road riding. It's tidy but needs some small jobs doing on it, one of which was a sticking front brake.  I stripped the caliper down, removed all the pistons and seals, cleaned it in my ultrasound cleaning bath, and bought a new piston & seal kit.... all good so far..... however I had a right job trying to get the pistons back into the caliper halves with the new seals in place. Everything was spotless and lubricated with clean brake fluid. I eventually got them in, however I soon found out that I'd nicked a couple of the  seals when fitting the pistons. When I came to bleed the brakes using my vacuum pump, it was sucking air in past the pistons ?  For info it's a AJP 4 piston caliper.

I've ordered some new seals and will try again, anyone else attempted this ? if so do you have any tips to pass on ?

 

thanks

 

Mat

 

 

Edited by matgriff
typo

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Use a small mirror to check the seal groves in the caliper as you can't see it all directly. Any muck or corrosion in the grove will make the seals too tight.

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Hi Mat,

I’ve only had to reseal 1 front caliber on my old scorpa sy250, but they have the same caliper (for the most part). I am by no means a specialist, but I found that splitting the caliper in half made it very easy to push in the new pistons using a vice with a soft plastic jaw on it (soft wood would work too). There is an oring that seals the two caliper halves if I recall correctly, and will likely be reusable when you put the two halves of the caliper back together. Just throwing out an idea if you haven’t tried it.

 

Another good idea I can pass on is to remove the pads from the caliper once you’re ready to bleed it, and put a piece of wood between the pistons - in case it leaks, it won’t contaminate your brake pads. Definitely block the pistons so that you don’t push them past the seals when bleeding (if caliper is removed from bike). I was so cautious bleeding my front brake and still managed to contaminate the pads with a very small amount of brake fluid from the bleeder nipple (not good, lol).

oh, and make sure you put the pistons in with the flat face inward. If put in backwards it won’t have the same surface area, and also I believe one side of the piston has a chamfered edge while the other side is flat and may be part of your problem as to why the seals were nicked?

I hope this helps. Disregard this message if you’ve tried all that. Good luck :)

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15 minutes ago, jackman290 said:

Hi Mat,

I’ve only had to reseal 1 front caliber on my old scorpa sy250, but they have the same caliper (for the most part). I am by no means a specialist, but I found that splitting the caliper in half made it very easy to push in the new pistons using a vice with a soft plastic jaw on it (soft wood would work too). There is an oring that seals the two caliper halves if I recall correctly, and will likely be reusable when you put the two halves of the caliper back together. Just throwing out an idea if you haven’t tried it.

 

Another good idea I can pass on is to remove the pads from the caliper once you’re ready to bleed it, and put a piece of wood between the pistons - in case it leaks, it won’t contaminate your brake pads. Definitely block the pistons so that you don’t push them past the seals when bleeding (if caliper is removed from bike). I was so cautious bleeding my front brake and still managed to contaminate the pads with a very small amount of brake fluid from the bleeder nipple (not good, lol).

oh, and make sure you put the pistons in with the flat face inward. If put in backwards it won’t have the same surface area, and also I believe one side of the piston has a chamfered edge while the other side is flat and may be part of your problem as to why the seals were nicked?

I hope this helps. Disregard this message if you’ve tried all that. Good luck :)

Hi thanks for the help. I split the caliper to clean it all out, the seal kit comes with a new O ring to seal the two halves back together. I also used a small piece of wood and a G clamp to push the new pistons back in (flat face inwards) So I was a bit annoyed with myself when I couldn't get them to fit. I made sure I drove them in square too.  The recess for the O rings is cleaned out perfectly, my ultrasonic cleaner fetched the caliper up like new.

On the plus side I found just the seal kit online for £7.50 so I thought I'd order two sets just in case... arrived today and each set contains ten seals, So I've got plenty of room for mistakes this time.

 

Mat

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1 hour ago, totty79 said:

Use a small mirror to check the seal groves in the caliper as you can't see it all directly. Any muck or corrosion in the grove will make the seals too tight.

Hi, thanks yes I'm sure they are clean, I put the caliper halves in my ultrasonic cleaner tank for 30 minutes, came up spotless.

In hindsight I used a bit of red rubber grease on the new seals, maybe that got behind the seals enough to stop them pushing right back into the grooves ?  I'll just use clean brake fluid next time.

 

Mat

 

Edited by matgriff

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Sounds like you’ll be up and riding in no time once your parts arrive.

The grease may have swollen the seals? I think you’re on the right path with just lubricating the seals/pistons with clean brake fluid prior to installation. Let us know how you make out :) 

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I have it figured out, the new seals are slightly smaller and thinner and fit correctly ? however the one i pressed home with the wrong seal was stuck and i had to damage the piston getting it out ... oh well the ebay seller who sent the wrong bits can send me a new set of pistons ... at least i have the right seals now 

3B5F24AC-8D60-4AB2-9F76-078B490251EE.jpeg

Edited by matgriff
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They are difficult to put back in but I wouldn't use a vice or clamp. Just lubricate with silicone grease and wiggle the pistons in by hand, if they are in correctly they will slide in with thumb pressure easily.

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Hi, yes I got it all done in the end. Warmed the seals and caliper with a hairdryer to soften the seals, then lubed them with clean brake fluid and twisted/pushed them home. Works fine now 🙂

 

Thanks

 

Mat

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