Simple, one of 2 things can cause this. If you suspect as previously discussed about no fluid return etc, then it would affect BOTH calipers equally, so that is NOT where your problem lies. Simple test to confirm that is to pump the lever multiple times to generate maximum system pressure, then release the lever. Now add a bleed hose to the left bleed nipple & crack the bleed nipple. If fluid squirts out under pressure then you have a NON return issue, if not then the caliper as detailed below.
#1
Pop the caliper off the fork then check that the caliper slides easily on its sliding pins. If not then seperate the caliper from the hanger & check the condition of the caliper sliding pins & the inside of the boots, plus the pads retaining springs etc. Is there lube present or corrosion ? Is the lube like treacle ? if yes then remove all lube from both the boots & slider pins. Then regrease with Silicone Grease or Rubber lube, do NOT use normal grease or oil, as whilst this will feel great immediately, it causes the boots to swell & grip the pins !!!!
Reassemle the caliper to the hanger & bolt back in place then pump the brake to seat the pads & check for drag. If OK job done. Suggest you repeat on the right caliper to keep their performance the same.
If its not #1 then it must be
#2
Remove the pads then pump the lever GENTLY, watch the pistons, they should BOTH come out at the same rate & return slightly when the lever is released, if not then the seal grooves will have corroded. If one piston comes out & the other does not, then use a socket or bit of wood to block the working piston & then try pumping the lever again. If the sticky piston comes out, check its outer surface for signs of corrosion, or seal material. Use a single pad against the opposite side of the caliper & pump both pistons out till they stop. Clean the outer surface with some WD40 etc, if its corroded then you may not be able to clean it in the caliper. Now lever the pistons back in & check the action, if its still sticky you will need to strip the caliper.
To strip, pump out as above but use a thin strip of metal to stop the pistons coming all the way out, but let them come as far as possible. Now protect the bike with some cardboard etc to stops spills touching any of it. Brake fluid will eat paint & plastics ! Use a plant water sprayer & damp everthing down as brake fluid is hydroscopic & will absorb the water & lose its nastyness ! Remove the caliper hanger & the rubber boot from the caliper.
Now you can unbolt the brake line & either clamp it shut or elevate it higher than the reservoir if possible & it will stop leaking, beware of fluid running down the hose & onto other things if you elevate it !
With the caliper off the bike, open the bleed nipple & drain as much fluid out of the hose connection as possible. Squirt everything with water whilst working to deactivate the brake fluid. now dry the exposed pistons & remove the stop plate you used to stop the pistons coming out. If you don't have a piston puller, then tape the outside of the piston & gently use some mole grips or pliers to grip & remove the pistons. Use a thin tipped screw driver to pop the seals out of their grooves, there should be 2 per piston.
Use some clean brake fluid to clean the seals & check their condition, if they are misshaped, then discard them & get some new ones. If they are the right shape i.e. round, then they can be reused. Check them for corrison adherence on their outer sides, if there is some, then a blunt blade can normally be used to remove it, so long as the surface stays smooth & undamaged when finished.
Now check the claiper seal grooves for corrosion, you may have to use a small allen key or piece of wire to run round the groove to feel for it. But usually you can see it. Best way to clean them is to use a dremel tool mini wire brush running in the grooves. If you don't have that, I have used a piece of bent wire slightly thinner than the groove & run it round the groove with some scrotch bright pad. Once clean, wash the whole caliper in hot soapy water then place in an oven set to 80-90c & dry it throughly. Leave to cool.
Now check & clean the outer surfaces of the pistons. If some of the seal is stuck to the piston, then use some scotch brite to clean it off, always work around the pistons surface, NEVER in the direction of piston travel, so that any micro grooves are contained by the seals ! Polish the pistons, but remember they are coated steel, so try NOT to remove the anti corrosion coating. If the piston surface is pitted, then clean them as best you can. Small shallow pits that are less than half the thickness of the seal surface should be OK, but if in doubt, replace the piston.
To reassemble, put a smear of fresh brake fluid in the inner fluid seal groove & insert the fluid seal. Then put a thin film of silicone grease in the outer dust seal groove & insert the dust seal. Now lube the fluid seal with fresh fluid & the caliper piston hole behind the seal, then lube the piston end & a little of the side with fresh fluid. Insert the pistons half way into the caliper. Now clean any visible fluid off the piston surface & coat the exposed piston surface with a film of silicone grease & push the piston all the way into the caliper. Now clean off any excess fluid/silicone grease from the dust seal. A properly serviced piston should be able to be rotated by hand in the caliper, it will be a bit stiff, but not to tight to move. Add the rubber boot for the sliding pin & assemble caliper hanger & pads back together. If you suffer from sqeeling brakes, then some copper ease or silicone grease on the back of the pads usually cures it. Use it sparingly, as you don't want it on the pads surface !
Now bolt the brake hose back on & mount the caliper. I have re-used brake banjo copper washers many times, your mileage may vary. If in doubt use new ones. Now bleed the brakes & Job done.