Could need a new o-ring on the lever piston, but if like you say the lever returns before the slave, sounds like another sticky piston (slave) issue.
I just drop a few drops of lube.. fairy liquid.. spit.. whatever onto the slave piston whilst the pad is fully out, and just work the lever loads so it lubes it up.
Also make sure both your pads are an even distance from the rim without any tpa on first.