As above, it seems that every time I leave my wee hardtail for any length of time the brakes, (formula k18s’) especially the rear, tighten up and are creating a fair bit of drag. I’ve tried pushing the pistons back in and the lever is solid. It’s like the system has been overfilled, is it possible that the fluid is absorbing water and pressurizing itself.
Anyone got any ideas or thoughts as to what’s going on?

Mine used to do that and the only way you could get a wheel to turn was carefully loosen bleed nipple and let some fluid out without sucking air back.
The long term cure was buying some SLXs.

Weird one… Shouldn’t be moisture unless there’s a leak, it could cause expansion like this in theory but in practice it’d need a lot of moisture. But if the brake’s “pumping up” then there’s only a couple of other things that could cause that, issues with the lever piston and issues with the diaphragm I suppose.

How old are they? An original Oro could be the best part of a decade old now…

Very common to find these brakes on peoples bikes in this state. The majority of owners haven’t flushed them in 3-4 years and this is usually what you find. Brake pad change is impossible without resetting pistons which means a heap more work than the likes of a shimano brake where you can reset the pistons into the bores without the possibility of bursting a seal at the lever.

Remove the pads,clean the edges of the pistons and lube them with hunters silicone lube (not brake fluid). Gently work them in and out, one at a time then back off a bleed screw before finally resetting both pistons flush in the caliper. At this point you can now start flushing the system a number of times as they tend to get fairly contaminated. Some brakes have fairly filthy fluid lurking at the caliper end (funnily enough) so if it’s possible, flush it out from the lever to the caliper to begin with to prevent dragging more crap through the entire system.
Do everything slow and gently. Don’t force syringes if you feel pressure building up in these brakes as you could damage seals. Take it easy and you’ll eventually get the fluid flowing easily with a bit of patience (if they’re in a bad state)

If you spend the time doing this, you’ll have a fully bled system that’s not over filled and you’ll be awarded with pads that clear the discs as well as they can, giving you a brake that won’t eat through pads as easily as there’s more clearance.

OK, thanks to all for taking the time to reply.
It’s true I’ve never bled or flushed these brakes in the 3-4 years I’ve had them, but in truth no other brakes I’ve had or have do this tightening up thing, if anything they lose pressure not gain it.
I will clean, flush, refill and bleed them, but would still like to understand what’s happening.