clutch failure on 1990 Chevrolet 3500 Pickup

My clutch hasn't been engaging properly so I bled the slave cylinder on the manual transmission.The clutch worked fine after that but soon it started loosing it's pressure at the pedal. I then replaced the clutch master cylinder and after a hour of bleeding the system I start to get pressure but the pressure start dwindling there after. There are no visible leaks in the system but the fluid looks milky coming out of the bleeder valve. Could the slave cylinder be defective? The pushrod on the slave cylinder has some rust and corosion on it. Maybe this is causing some air to seep through it's seal.

Asked by for the 1990 Chevrolet 3500 Pickup
I tend to replace the master and slave as a set. When one goes, the other is soon to follow, kind of like a pair of shoes.