def air in the system... probly in the master... vaccum bleed it or bleed it in sections..
master first then caliper
They can be a bugger mate. Pull the hose and resorvoir cap off the master cylinder and the hose off the caliper and remove bleeding nipple on caliper also.
Fill both sections on the resorvoir with fluid and fill the caliper with fluid also(both holes)and replace and lightly tighten the bleed nipple only.
Install the hose on the resorvoir and tighten, do not put cap on the resorvoir so you can fill it with fluid.
Hold your fingers over the caliper end of the hose and make a good seal(with fingers, keeping the hose higher than the resorvoir), as you push the brake pedal down you want to let your fingers loosen slightly letting air out of the line(do this process in between pushing down and reaching the bottom of the pedal, as you dont want to suck any air in)
When you start getting fluid coming out and whilst bleeding the line and starting to feel pressure you may keep your fingers tight on the end of the hose(keep an eye on fluid level and fill accordingly), when pressure is at a maximum you will not be able to hold the hose(keeping fingers tight)without fluid escaping due to full pressure, whilst pushing on the brake pedal.
When you feel this has happened, install the banjo bolt on the end of the hose and quickly install on the caliper.
Your line should be 99.9% bled, check all fittings are tight and that the resorvoir is full and install resorvoir cap.
Pump the pedal and prime the caliper until you feel the brake pedal stiffen and move the wheel with brake on to check functionality.
If your happy with the brake then leave it(only if your brakes are locking with you sitting on it and moving back and forth), if they are a little spongy you can then bleed like a car brake system, or if all has gone to plan you can crack the bleed nipple(remove resorvoir cap for filling) and the fluid should run(gravity bleeding)and some air bubbles should be present at the nipple, but if you get brakes and think there good leave them, not all brake system bleeds are this complexed but for no reason some are an absolute nightmare, this sounds like your prob. Im tired