I've just put new pads on shimano deore disc brakes for the first time. I pushed the pistons right in until flush, popped in the new pads easily, but when I pull the lever the pads stay against the disc. It's a slight but continuous rub at the front, but the rear barely moves.
I'd recently flushed with worn-out pads, so I thought possibly there was too much oil. I pushed the pistons back in allowing excess to leave, but same problem.
I recently did a long drive with the bike on the back exposed to rain and salt which I don't think helped. Any suggestions?
Deore disc brakes (and most others) use a spring to apply pressure to the pads which helps the pistons and pads retract when you release the brakes (the primary return mechanism for the hydraulic system is a spring in the piston assembly in the lever, however as this only acts on the pistons the pad return spring is responsible for separating the pads from the rotor). Often a pad return spring will come with the new pads (an example listing from CRC) but if not you can reuse the old spring as long as it is undamaged.
If you don't recall installing the spring when you changed the pads it may have fallen out and gotten lost, or it remained in the caliper when you removed the old pads and is now sitting underneath the new pads. When installing the new pads for this style of brake the prongs of the spring should sit either side of the friction material and rest on the face of the metal backing plate closest to the rotor (~5:30 in the linked video shows the process). The holes for the retention pin should line up as well.
If you did all of the above answers and it's still not quite right.
The next question would be have you ever changed your brake fluid?
Depending on riding style, conditions, etc. usually by the time you need new pads on a mineral oil type brake it is also time to change the fluid.
This is something that if you don't have quite a bit of experience in doing should be left to a pro.
Note- not all bike shops have "pro" mechs you might need to shop around to find someone who actually knows what they are doing.