I have a pair of M785 Shimano XT’s I am putting on a new build. When I bleed the brakes I use shimanos bleed kit with the reservoir and bleed block. With the bleed block and reservoir installed the brakes feel extremely firm and engage quickly. But when I take off the reservoir, re install the reservoir screw and put the caliber back over the brakes and rotor the brakes become spongy and need a lot more travel before engagement (the engagement is not very firm either). I compared the thickness of the bleed block to the, but hard to get a good comparison while including the rotor width.

So is this a case that the bleed block is thicker than the pads and rotor?

Probably, bleed on the bike for better results

and to qualify that – I’d rather the brakes worked properly as they are going to be used than in the workshop. The size of the brake block doesn’t really come into it.

EDIT:
I just realized that the brake pads have a spring that pushes them back apart when the lever is released and that is why they feel “spongy”. So perhaps the brakes springs just need to get some use on them (new bike build).

That doesn’t make any sense, they should feel firm, when you pull the lever it should have a stop at the bite point. The force of the spring is tiny

Perhaps I am misunderstanding you then. If you say that the size of the bleed block doesn’t matter, then I should get the same feel with the bleed block as I do with the pads/rotors. But I don’t. So I don’t 100% follow how bleeding with the pads and rotor in will improve the braking.

No, the size of the block doesn’t matter as you want them to work on the bike, set them so they work on the bike not the block – thats where you want better performance.

Ah so are saying throw the block out and keep the pads one when I bleed the brakes. Guess I better get neater with my bleeding.

It has it’s place, but if you can’t get them bled properly with it then I’d move to the on the bike method. Do you have new pads in there?
The other thing is to make sure the reservoir is properly full – a bit more fluid after you take the funnel off helps.

New pads. I have been filling the reservoir to the brim when I thread the screw in.

Thanks for the advice!

New pads. I have been filling the reservoir to the brim when I thread the screw in.

Thanks for the advice!

New pads. I have been filling the reservoir to the brim when I thread the screw in.

Thanks for the advice!

Also wind the free stroke screw out as far as you can, plus have the reach adjuster wound out so the levers are as far from the bar as possible.
This lets you get a little more oil in the reservoir.

I’d also suggest checking your caliper alignment, if both pads aren’t hitting the disc flat, square, and at exactly the same time then you’ll be putting your effort into distorting the disc/trying to cock the pistons in their bores, and will be getting a spongey lever amongst other bad things.

With that generation of XT I found that using the yellow block whilst bleeding gave more lever throw than I liked. I just took a file to the block to make it slightly narrower.

They shouldn’t feel in any way “spongey” though – that’s generally the sign of a poor bleed.

And always genuine Shimano mineral oil.

I usually find a dominant piston is the problem

With that generation of XT I found that using the yellow block whilst bleeding gave more lever throw than I liked. I just took a file to the block to make it slightly narrower.

+1. tale a mm or so off the block and it sets up so much better. Bleeding with pads and wheel in runs the risk of the hose popping off the bleed nipple and contaminating

I’m with coatesyand wysywig. Dominant piston causing flex of the disc = massive spongey feeling.

Yup have experienced the lazy piston syndrome quite a few times. It doesn’t take much – fractions of a mm – to turn XTs into badly adjusted BB7s

If bleeding with the block is leaving the pads too far from the discs then whip the wheel out, squeeze the lever a couple of times and pop the wheel back in. Rinse and repeat till close as you want and then check alignment. You could also top up with oil at the lever port at this point in a similar style to the “Five minute bleed”; http://www.epicbleedsolutions.com/blog/5-minute-shimano-mini-bleed/

Also wind the free stroke screw out as far as you can, plus have the reach adjuster wound out so the levers are as far from the bar as possible.
This lets you get a little more oil in the reservoir.

Never though about the reach adjuster giving extra oil for the reservoir, good idea.

With that generation of XT I found that using the yellow block whilst bleeding gave more lever throw than I liked. I just took a file to the block to make it slightly narrower.

I think this is my exact problem. I thought about it and I think the problem with the block is that when braking that pads compress thinner than the block, so that is why is feels different.

I usually find a dominant piston is the problem

I have never heard of this issue. how do I go about confirming that it is a dominant piston and solving it? When I look at the pads closely while squeezing the lever, the pads looks like they compress together and evenly.

Thanks for all the help!

Loosen caliper bolts, pull on brake, tighten caliper bolts, release brake, have a look and see if if one side or other has more less/no clearance. If one side is still against or closer to one of the pads the other piston is ‘dominant’ or moves more, basically. Just bias the alignment so the piston that moves te least is closer to the disc. (pretty much as it will be once you’ve done the above procedure).