Release cylinder - sounds like that means the slave cylinder...

But actually, your problem sounds more like the master cylinder. If the slave were leaking, you'd be able to see it & the fluid level would drop.

When master-cylinder piston seals leak, the fluid doesn't leak OUT, it just bypasses the seals. When this happens to your brake master your pedal sinks while you're stopped. On a clutch, it slowly engages your clutch while you hold the pedal down.