Tektro HTC 300 hydraulic fluid leakage
If you watch the Tektro video you will see that some fluid leaks from the bottom bleed port threads as the new fluid is injected.

If all the fluid is leaking out, then that might indicate a blockage. You may just be getting more leakage than expected because you have to refill the entire system.

The fully retracted piston should not be blocking the system.