question re: Shimano yellow bleed block spacers/pistons

[eddief]

I am by now a half-way decent wrench but have been getting pissed off when I try to bleed by disc brakes. When I remove the pads and try to stick the bleed block spacer between the pistons it never goes in well. I have the special tool to spread the pistons but confused about how to use it.

So far I have cracked a piston in two different calipers. Replacement calipers are in radically short supply and when available are expensive. And best I know piston rebuild kits are non-existent.

1. When you apply the spreader tool, do you leave the pads in and stick the tool in between the pads?

2. Or do you remove the pads and use the spreader against the bare pistons?

3. Regardless of which I approach I use, it seems like the pistons do not back out enough to allow getting the bleed block in there.

A friend suggested this is a common issue, ceramic pistons are prone to cracking and suggested grinding down the bleed block so you can get it in easier.

What say you?

[NHAero]

I take out the pads, to be sure, before using the yellow Shimano bleed block, and I haven't then noticed difficulty getting the block in. I have the Park tool that levers the pistons back.

If a system is slightly overfilled with fluid, I can envision that the pistons don't move back enough. But I'd expect with that condition that pad rub would be an issue too.

[MikeD]

What's this spreader tool? If it's metal, you should never use a metal tool on a ceramic piston. A mechanic in another forum installs worn out pads and and pushes against them to push the pistons back.

[Hank Scorpio]

You could crack open the bleed fitting on the caliper itself as you push in the block. I can’t imagine that it would be that right but possibly if the pads are extra worn the pistons may be further out than they should be.

[eddief]

https://www.biketiresdirect.com/prod...UaAtbbEALw_wcB

Maybe using this tool with an old set of pads installed is a good approach.

Originally Posted by MikeD

What's this spreader tool? If it's metal, you should never use a metal tool on a ceramic piston. A mechanic in another forum installs worn out pads and and pushes against them to push the pistons back.

[eddief]

something as simple as this:

https://www.youtube.com/watch?v=4C9R_H_GLKI

[Upcountry]

A Pedro's Tire Lever works really well, as its just the right width and thickness to get in there and leverage the piston back to its recessed position.
My first thought is that you have too much fluid in the system? I can pretty easily push my pistons apart enough to get the yellow block in there, without opening a bleed port, but perhaps attach your upper funnel in the bleed port and then press the pistons...

[eddief]

My latest attempt was on my road bike where I am installing upright bars. This required replacing Di2 STI shifters with new flat bar Di2 shifters and new mountain brake levers. I am sure the levers came already full of oil...and only a tiny bit of oil dripped out of the hose end when I cut off the old barb and re-installed the hose into the new brake lever. Soooo maybe I WAS trying to force the pistons in against a too-full system. goddammit.

I just assumed the pistons would go in some no matter what.

Thanks for the tip. Two new calipers and two trips to the LBS to deal with these mistakes has been very costly. I wanna get it right.

Originally Posted by Upcountry

A Pedro's Tire Lever works really well, as its just the right width and thickness to get in there and leverage the piston back to its recessed position.
My first thought is that you have too much fluid in the system? I can pretty easily push my pistons apart enough to get the yellow block in there, without opening a bleed port, but perhaps attach your upper funnel in the bleed port and then press the pistons...

[tkbike]

Originally Posted by Upcountry

A Pedro's Tire Lever works really well, as its just the right width and thickness to get in there and leverage the piston back to its recessed position.
My first thought is that you have too much fluid in the system? I can pretty easily push my pistons apart enough to get the yellow block in there, without opening a bleed port, but perhaps attach your upper funnel in the bleed port and then press the pistons...

Yup, tire levers have worked for me since the original formula brakes I used in the late 80's early 90's.

[bigbill]

I use the handle on a Park cone wrench. My gravel bike is post mount so I have to loosen the mounting bolts to allow a wiggle room to get the block installed. the rear caliper tends to stick and I figure I'll eventually break a piston. A year ago I bought a pair of 785 calipers to keep on hand.

[Velocipede]

Your issue could be the bleed block itself. There are 3 different yellow bleed blocks from Shimano. And two of them are incredibly close in size. You might not have the right block. If you do have the correct one, expand the pistons with the pads in the calipers. That way you don't damage the pistons (they are ceramic and will break easily, the pads won't). If you do that, and have the correct block, you shouldn't have any issues getting the block in the caliper.

[eddief]

thx

Originally Posted by Velocipede

Your issue could be the bleed block itself. There are 3 different yellow bleed blocks from Shimano. And two of them are incredibly close in size. You might not have the right block. If you do have the correct one, expand the pistons with the pads in the calipers. That way you don't damage the pistons (they are ceramic and will break easily, the pads won't). If you do that, and have the correct block, you shouldn't have any issues getting the block in the caliper.

[txsurfer]

I'm with others. Tire levers and open the bleed valve. Works every time for me.

[tellyho]

Huh, I've never cracked a piston. Hayes used to recommend a 10mm box wrench, but with ceramic shimano pistons, Pedro's levers are better. That said, I've totally used just a flathead with shimano and never cracked one. I agree that opening the bleed fitting on the reservoir would be a failsafe.

[peanutgallery]

Old pads and flat screwdriver as stated above. I can't get enough leverage with a plastic tire lever and since you're probably replacing the pads anyways