Hydro clutch giving out

[62 Barnfind]

I took my 62 in to the muffler shop today and when I was loading on the trailer the clutch went out. I lost the pedal and it went to the floor. I pumped it up and I got it back. It works fine until I keep my foot on it and then it gives out. I think it's the either the slave or master cylinder. I don't think it;s the clutch because it works until I hold it down. I like to ask all of you before I start troubleshooting because most of you probably had this already. What do you all think? Feed back is great!