I need to shorten the rear hose on some brakes I’ve just bought and I wanted to know whether there was a good technique to ensure I don’t have to bleed the brakes afterwards as the mineral oil is expensive!

So, what stops all the fluid pouring out of the master cylinder when the hose is out? Air pressure? Just keep the refill hole closed, cut refit with new olive tighten in?

Just keep the refill hole closed, cut refit with new olive tighten in?

Pretty much

Epic bleed solutions has a great guide on this.

EDIT. Find it here: http://www.epicbleedsolutions.com/blog/shorten-shimano-hoses-without-bleeding/

So, what stops all the fluid pouring out of the master cylinder when the hose is out? Air pressure?

Viscosity!

You’ll let a tiny amount of air into the system, but if you lightly tickle the lever for a bit after doing it then you’ll send it into the reservoir.. Put your ear to the master cylinder and you’ll hear when the slurping stops. Get a small bottle of oil, and a bleeding funnel and you can remove that bubble without having to bleed the whole brake.

I found it also helps if you rotate the lever so it’s on top of the bar as well.

And don’t, whatever you do, accidentally knock the lever before reconnecting the hose like I did….. 😳

What the pinkster said not that I did it twice in an evening (oh alright I did).

Be flipping careful when you put the hose back in the lever to make sure it’s properly in there before tightening the nut.

+1 on what garage dweller said, make sure the hose is back in that lever straight and tighten slowly. My rear brake went fine but on the front the olive didn’t go in perfectly straight and didn’t seal.

You want to ignore bike whisperer, its not viscosity, nor do you want to be using that method to get the air out.

If you must bodge, pull the lever a few times to push the pistons out as much as your dare , do your hose cut , refit , stick in funnel and push the pistons back , top up fluid.

Personally shimano factory bleeds are hit and miss and i ended up bleeding every single shimano brake that left the shop as it was easier than having to deal with a customer whos brake had gone spongy or gone all together due to poor factory bleeds.

Thanks all, will do it the trail_rat and Epic Bleed Solutions way.

It’s air pressure. Other than the hole where the hose connects, there’s nowhere for air to get in. If the fluid came out it would, in effect, create a partial vacuum in the reservoir. We switch brake hoses on most new bikes as the come “Euro” style. I guess it’s about 1:50 that needs a bleed/top-up once the hoses are reconnected.

Trail rat, you’re right.. It’s surface tension, not viscosity. Teach me for not thinking while I type!

The lever moves around half a ml of fluid though.. Far less than the volume of the hose. Pushing the pistons out loads doesn’t make huge odds on this.

^^ scotroutes

partial vacuum in the master cylinder

as long as you don’t do anything stupid like shake the bike / turn the bars, or pull the brake lever you should easily be able to cut the hose, install the new olive / barb and reinstall the hose without needing to bleed the brakes

have done this for Hope, Formula, Avid, Shimano, Tektro for years without needing to bleed

of course, if a sales assistant in our shop walks into the workshop and decides in a moment of clarity to pull that brake lever, dumping the fluid onto the floor and sucking air into the MC, that’s another matter…