2001 ranger bleeding clutch

[eric]

I was told by a ford tech that the only was to bleed is to open reservoir, leave fluid in , pump clutch 15x then hold clutch petal down 10 seconds. Repeat for 30 minutes. I have done this and clutch works great for about 15 minutes then I start losing it again. I had a slave cylinder put in 10 months ago. Could this be a case of lots of air in system and needing a heavy bleed?

Thanks in advance