One of the seals fitted wrong way round maybe?
If not then take the two bolts out of slave cylinder and let hang on hose.
Push the piston in as far as possible, you may have to take lid off M Cyl.
Put a bolt down inside the slave cylinder to rest on the piston.
Use G clamp or strap to hold the bolt in place.
Bleed until solid.
Should remain solid.
If not solid is fluid leaking anywhere?
If no leaks but pedal spongy I'd try a different M/Cyl..... Maybe pipe it up to the brake one temporarily.