Anyone know anything about disc brakes?

Yes. Also any DOT 4 or 5.1 brake fluid will do. All you need is a new clean syringe

Can you see where the oil coming from? Maybe a rebuild kit.

It's around the pistons somewhere... bleed port and hose connector fine

I've got a set of RX4 in a box I've been meaning to sell...

Version/colour?

Black / Shimano flat mount

what's the "size" on them... there's 2-3 variants for both front and back I seem to recall

So possible seal and piston needs replacing. Not a big job.

Quick search doesn't come up with any suppliers tho. Merlin cycles do the deore complete set up for 50.

It'll be a cracked piston, Shimano don't do spares, so grab a new caliper, any flat mount one will work, spend more for slightly fancier. The deores will be post mount not flat.

Is a cracked piston common?

On Shimano brakes with ceramic pistons, yeah.

Never understood why Shimano don't sell replacement pistons and seals for their brakes; they do replacement parts for almost every other thing they make

I mean, even SRAM are able to do it, and their name stands for Spares Readily Available, Maybe.

Now just talking, but that sounds like an inherent fault to me.

I'll probably replace the oil, as it's been in for years, riding in slop (it's an MTB). Or can I get away with just getting rid of the air?

I took the old, worn pads out and then pulled the brake lever to push the pistons out to clean them. A little oil came out around one piston. When I try to push the pistons back in, they always pop back out a little bit which means there's not enough clearance for new pads. I assume there's air in the line now due to pushing the pistons too far out, as well as the oil being dirty and there not being enough of it after the little leak. I hope I haven't broken the bladder or reservoir or whatever it's called by pushing too hard on the pistons trying to force them back in.

There's an amount of user error pushing pistons back in wonky, but also some that just break as they're a bit fragile.

Not sure how to get all the air out without replacing all the oil. As the system is such a small amount of fluid.

Also are you replacing like for like fluid? Different colours?

Is it because the pistons are cubic zirconia reinforced ceramic so weak in certain directions.

I will put DOT 4 oil in, I don't know what's in there at the moment.

That is a good idea, as dot brake fluid absorbs water. Not as if the there is lots of fluid.

Careful as dot brake fluid ruins paint and skin etc.

EDIT 355ml of brake fluid is 4.49 in Screwfix/Toolstation while 125 of Halfords stuff is £4.99

Careful as dot brake fluid ruins paint and skin etc.

Not if you wash it off with water

True, but better not spill the stuff. Then have to throw lots of water around.

Also contact dermatitis is not fun and best avoided.

Any recommendations for a good barb press that isn’t loads of money?

AliExpress. I got one for about £4 which works just as well as any other one I've used. And actually the built in hose cutter is better.

I've been very happy with this one. I ordered from Amazon as I needed it in a hurry. I'm sure you'll find the same product for cheaper on AliX if you're happy to wait a little longer.

DONGKER Bicycle Hydraulic Hose Fitting Insert Tool,Needle Driver Insertion Tool w/Hydraulic brake Hose Cutter for Bike Road Bicycle https://amzn.eu/d/cp8D5F6