Hydraulic brakes, cold causes 0% braking capacity Its recently been cold, around 0 degrees C at night, where my bike is stored in the outside shed.

Ive replaced the pads, and bled the brakes around 6 months ago.

When taking out my bike it has 0% brake capacity initially, and the levers touch the handle bars without stopping the wheel.

After pumping them for around 5 minutes, they sharpen up to around 40%.

Then they jump between working (40%) and not (0%) randomly over the next 10-20 minutes.

What could the issue/fix be?

Should I re-bleed the brake fluid, replace the o-rings, or keep the bike indoors?

Should I be able to store my bike in a shed at low temperatures without the brakes breaking?

Thanks
I would say that you've either got air in the lines or perhaps moisture in the brake fluid. Assuming that mechanically the brakes are good.  I would completely drain the brake fluid and replace it with the manufacturers spec fluid and bleed it properly.