They're OE brakes I believe. The Olive is inside the brake already - under the yellow plug, however if you want to trim the hose (Theyre supplied at normal 'full length') you'll either need a barb or cut the existing one from the loose bit of hose.

These are sold like this do people can thread them through frames with internal routing, and then attach the hose/lever.

Olive is in the lever already.

Cut hose to desired length, reuse barb, and then push hose into lever and tighten the nut.

Thanks Ben.

Will the olive fit onto the hose with barb already inserted ?

Do I need to remove that rubbery material?

Can I re use the barb if I need to shorten the hose ok then ?

Yes, barb can be reused ... don't cut your fingers off liberating it though.
Yes don't chuck the rubbery material... at some point it will come in very handy.
It might fit but it's unlikely you won't want to cut the hose.

Will the olive fit onto the hose with barb already inserted ?

Do I need to remove that rubbery material?

Can I re use the barb if I need to shorten the hose ok then ?

Sometimes, sometimes the olive is jammed in the lever sideways.

Sometimes, sometimes the sealant (that shimano say we do not need to use, but they use...) blocks the lever.

The barbs are not designed to be re used, but if you can carefully remove it from the hose they can be re used. A new olive and insert is £2.99 RRP IIRC.

Have you got everything to fill/bleed the system after faffing with hoses and barbs? If not I'd definitely add that to your shopping list.

I'm pretty sure you'll find a new barb in the baggie with the rubber hose boot, bolts and "anti-undo" clips.

I've not had much luck with the "j kit" as as soon as I pull the bung I find the master cylinder all but empties itself in the time it takes to push the hose inside and tighten things up.
You'll almost certainly need to fully bleed once you've attached the hose to the master, however I can happily cut the hose at the master cylinder and do an internal hose install on a pre assembled brake and only need a master cylinder bleed to finish.
I think I must be doing something wrong with the j kit. Eehaps I should RTFM 🙂

Trim hose, get someone to hold it upright while you fiddle with the barb removal, carefully slice the outer layer, peel off then a lighter does the trick by softening the inner hose to free the barb with a little pull.

The barb should be in with the bolts and bleed blocks/hose clamps. Along with a spare olive (there will also be one in the lever body.

I did a set like this recently, I know it’s radical - but I just went on the shimano site and read the manual, it has pictures and everything.

Will the olive fit onto the hose with barb already inserted ?

Do I need to remove that rubbery material?

Can I re use the barb if I need to shorten the hose ok then

Without wanting to sound rude but if your asking these questions and your not really sure,  should you be setting up your own brakes?

It might be worth booking this into the LBS. Attach the caliper and lever and route the hose and just ask them to attach the hose and bleed. Depending where you are in the UK it's about £25 a brake to bleed, give or take.

It's not a hard job to bleed/trim hose to correct length. If shops are charging £25 a pop then even more reason to have a go. You can always take it to one if you don't have any joy, but take your time and it should be fine.

I do remember seeing a Pro team mechanic's vid on youtube (can't remember the name) where they filled the funnel and opened the bleed valve on the caliper and let gravity do it's job rather than fill from the caliper up into the funnel.
I've not actually done this yet but think others on here have done it this way with good results?

I got a set of (boxed) XTR brakes - very similar setup, there was a spare barb in the box though, so I'd look for that, and as @IA points out the Shimano manuals cover all this.

It’s no harder than threading in a BB ............ 🤭😂

This is the first set I've bought that have come like this. They normally come all connected up so shortening the hose when like that is a piece of cake.

However I've not seen a set like this before so it kind of threw me a bit and didn't realise they were known as j kit.

Merlin website doesn't make reference to it being like this.

@Letmetalktomark ..... You could be right 😜😂😂

I do remember seeing a Pro team mechanic’s vid on youtube (can’t remember the name) where they filled the funnel and opened the bleed valve on the caliper and let gravity do it’s job rather than fill from the caliper up into the funnel.
I’ve not actually done this yet but think others on here have done it this way with good results?

Sure it was Jason Marsh, ex Santa Cruz Syndicate/Greg Minnaars mechanic, I've used both methods but used an Epic Bleed Solution kit instead of a Shimano (open syringe instead of a funnel but essentially the same idea). Both worked fine

I do remember seeing a Pro team mechanic’s vid on youtube (can’t remember the name) where they filled the funnel and opened the bleed valve on the caliper and let gravity do it’s job rather than fill from the caliper up into the funnel.
I’ve not actually done this yet but think others on here have done it this way with good results?

Sure it was Jason Marsh, ex Santa Cruz Syndicate/Greg Minnaars mechanic, I've used both methods but used an Epic Bleed Solution kit instead of a Shimano (open syringe instead of a funnel but essentially the same idea). Both worked fine. Pretty sure he also recommends removing the pads and pumping the piston out so far then using the brake fluid to lube them before pushing back with a tyre lever and re fitting the pads, obviously cleaning any excess oil away first

I bought some J kit brakes from Germany last year and took my time, followed the instructions carefully and didn't need to bleed them. I seem to remember setting up my Black and Decker workbench next to the bike to help me keep everything vertical and so I had everything within reach.

There were definitely spare barbs and olives and an olive preinstalled in the lever.

Cheers. I will go and double check in a bit.