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I'm trying to replace the front pads on my 82' yamaha seca 400. I bought new pads and they seem to match exactly to the old ones just more pad to them cause they're new. But I can't seem to fit them into the caliper now, they are too thick. and the piston doesn't seem to want to push back any and I'm afraid to force it. any ideas?
 

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There's probably a "crud ring" on the piston that needs cleaned-off. Take the pads back out of the caliper, GENTLY give the lever just the tiniest of squeezes to push the piston partially back out of the bore (a small piece of wood might help you not to "overshoot" it), then use Brake Parts Cleaner and an old toothbrush or brass-bristle brush to thoroughly clean the piston(s) of any spooge. Crack the bleeder-screw, then push the piston back-into the bore.

Carefully inspect any pins for corrosion/straightness when you reattach your new pads.

Bleed the caliper with fresh DOT3 or DOT4 (I like the DOT4 even in DOT3 applications - it's compatible, but NOT the other-way 'round!), and you're done.

It goes without-saying, you should protect your eyes during any/all operations where some chemical or debris might be flung-into them. That Shyte Hurts.
 

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The Toad
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There's probably a "crud ring" on the piston that needs cleaned-off. Take the pads back out of the caliper, GENTLY give the lever just the tiniest of squeezes to push the piston partially back out of the bore (a small piece of wood might help you not to "overshoot" it), then use Brake Parts Cleaner and an old toothbrush or brass-bristle brush to thoroughly clean the piston(s) of any spooge. Crack the bleeder-screw, then push the piston back-into the bore.

Carefully inspect any pins for corrosion/straightness when you reattach your new pads.

Bleed the caliper with fresh DOT3 or DOT4 (I like the DOT4 even in DOT3 applications - it's compatible, but NOT the other-way 'round!), and you're done.

It goes without-saying, you should protect your eyes during any/all operations where some chemical or debris might be flung-into them. That Shyte Hurts.
After cleaning the pistons you can use a C-clamp to carefully push the piston all the way in. The brake caliper/piston assembly is quite hardy so don't worry about damaging it as long as you don't torque it hard.

You should remove the brake master cylinder cap and remove out some of the fluid before you compress the pistons. Compressing them back into the caliper will push fluid back up into the reservoir and if full it will overflow. But this way you don't risk getting air into the system.
 

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I've never had trouble pushing the piston(s) back in with my hand (on a Moto), unless something was buggered. In that case, it needs to come-apart completely. And, I have this aversion to pushing the nasty, blackened, used-up fluid back UP into the system.

I've worked on a few too-many ABS systems where this WILL damage them, so I've gotten-into the habit of cracking the bleeder-screw and pushing the old fluid out through there on every system.
 

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The Toad
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I've never had trouble pushing the piston(s) back in with my hand (on a Moto), unless something was buggered. In that case, it needs to come-apart completely. And, I have this aversion to pushing the nasty, blackened, used-up fluid back UP into the system.

I've worked on a few too-many ABS systems where this WILL damage them, so I've gotten-into the habit of cracking the bleeder-screw and pushing the old fluid out through there on every system.
Yeah well I replace the fluid when it starts to discolor so worrying about a lot of crap in the system is off of my radar screen. It only takes a half hour to do both systems on most bikes. Pretty easy really. 'Course few people bother with it.
 

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You might try the "primal scream-large hammer" technique, usually effective in these types of situations.

If nothing else it keeps the neighbor kids from pestering you while you're in the garage......
 
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