how do tell if your brake booster is bad?

[modifiedGMC]

That may sound like a dumb question but I just mounted a power brake booster on my 69 GMC (it had manual 4 wheel dsic brakes). I have ran this 4 wheel disc setup with manual brakes for the past year but just wanted to go with power. I got everything mounted hooked up and started bleeding the brakes (engine off). The brakes bled off good so I cranked it up to see how it felt but it didn't feel like power brakes, it was easier than engine off but was still kinda stiff like. I took the booster off of a 86 IROC and lengthened the brake rod going to the pedal. The master cylinder and prop valve are off a 79 caddy that I got the rearend from. It has front and rear disc brakes. I don't know if the booster is bad or something wrong with master cylinder. Is there any way to check booster? I have the vacuum line hooked up right, and there is vacuum going to the booster. Any ideas?