joegoersch

I accidentally squeezed brake levers on Shimano Disc brakes (BR-RS785) without the block in (while cleaning up after filling system--yes, I'm an idiot). The pistons are sticking out now and the brake lever mushy. I've read to take a plastic tire iron and just push pistons back. Should this be done with bleed screw open to relieve pressure ?


Thanks !

Dan Burkhart 

Shimano Hydraulics are an open system. as long as the pistons have not gone past the point of no return, they can be pushed back. The fluid will flow back to the reservoir. Don't open any bleed screws.

joegoersch

Thanks.


Going past the point of no return sounds bad--I hope I didn't do that !


I'm going to try to find schematics of the system so I know what you mean when you say "Open System." Does it just mean that when brake lever not depressed reservoir and line are in continuity ?

Dan Burkhart 

An open system resembles an automotive type system with a reservoir to draw fluid from and for fluid to return to. That's not a complete explanation of how it works, but that's the basics.
A closed system has just a master and slave piston with a closed line between them.

SquidPuppet

Make sure the reservoir isn't full before you push the pistons or you'll have a mess to clean up.

joegoersch

Thanks. Will do when I get home. Work really interferes with my bicycle maintenance....

hagak

Curious on an open system what keeps air from getting into the system? Is it just gravity and this is why you should not hang them upside down?

SquidPuppet

Yes and yes.

TimothyH

Anyone know if these calipers can be rebuilt in such a situation?


-Tim-

gsa103

Rebuilt how? Shimano doesn't sell any spares, so the only thing you could potentially do is replace the square O-ring.
If you do push the pistons out too far you can usually just push them back in, and the refill & bleed.

fietsbob

...or rebuilt? Why ? Take the pads out , so you dont mess up the friction material , pry the pistons back out, put the pads back in..

I pried the pistons out wider when I replaced the disc brake pads on my old car, many years ago..

San Rensho 

Dick breaks are the best, aren't they? Jeez those worthless caliper brakes never worked and were impossible to work on, right?

InOmaha

I've accidently done the same with a car. I forgot to put the passenger side caliper back on before attempting to bleed the driver's side. I pushed everything back in and put it on. That was several years ago and no leaks.


So I'd do that and look for problems after.

joegoersch

Canker

Everybody really wants to over complicate this for some reason. Just push the pads back apart. This isn't a big deal.

gsa103

Hanging up-side down is fine. The reservoir is not connected to the lines when the lever is open. No fluid flows when you open the bleed port. If you turn the bike upside AND pull the lever, that could potentially introduce air.

Additionally, there shouldn't be air in the reservoir, since it's nominally full, with a rubber seal that takes up space when the pistons move inward.

TimothyH

The OP says the pads weren't installed. Neither was the bleed block.

The problem is that without the pads or block installed the piston can pop out of the bore. That is what the OP is worried about.


-Tim-

TimothyH

For the same reasons and the same way an automotive caliper is rebuilt.

Since it is apart and the pistons are out, might as well hone and replace the seals, especially if the calipers have some miles on them. I've rebuilt auto calipers. Rebuild kits are cheap.

It was just an offhand question and I just realized that auto calipers are open on one end so the hone can enter the bore. Not so a Shimano bicycle caliper. Even so, it wold be nice to be able to replace the seals and reuse.

@fietsbob, I've seen someone push the piston back into the bore with pliers and cock the piston so that it stuck in the bore. It has to be pushed as straight as possible into the bore. I don't trust those piston tools sold by bike tool companies and wouldn't use a screwdriver for this reason. I know lots of people have done it that way but I'd stick a flat piece of metal, something like a ruler, into the space and press evenly on both sides


-Tim-

gsa103

Avid sells rebuild kits for their calipers, Shimano prefers you just buy new.
The main thing is that bike calipers see much much lower usage than a car. A car would have several thousand hours of run time before it hits 100k miles. The average bike gets the equivalent of 10-15k miles (hundreds of hours). By the time the pistons or seals need work, most people have moved on to the next bike.

This does have implications for 10-20 years down the road, when disc brakes will be on bikes that aren't worth the cost of new calipers. I suspect you'll see a small cottage industry of someone selling Shimano replacement seals.

joegoersch

No big deal. Plastic tire iron and calipers went in easily. Piece of cake. But now when I depress the brake levers oil leaks out the bleed boss. I loosed and retightened the bleed screw (firm, but not over torqued) and fluid still comes out the bleed port on the calipers. I would think the bleed screw should stop this. Defective bleed screw ? Bigger problem ?

Note: this was original filling of oil, so I don't know that the system was ever working--brand new.