front disk swap uncertainty

[68lngbed350]

I upgraded my front brakes to disk on my 68 3/4 ton recently. I got the whole kit from a truck at my local junkyard including the used booster and used master(from that same truck). I bled everything and I took it for a test drive. The brake pedal has a very spongy feel, comes back up slowly when released. With alot of brake pedal effort the truck barely rolls to a stop. When I pop the hood with the engine running I can hear a hissing sound from the booster area. I confirmed the vacuum hose is not leaking. At this point all I can think of is a bad booster and/or master. I bled the brakes numerous times and checked for leaks throughout the whole system.

Suggestions? Tips? Any feedback appreciated