Can you lock up your disc brakes?

Curious if anyone with 4 wheel, manual disc brakes can lock up the tires under hard braking? I have 13.75" rotors with Brembo 4 pistons up front, mustang cobra discs in the back. 15/16" bore master cylinder, and adjustable prop valve. Although, the valve is completely open and not regulating any pressure to the back brakes. Also, the brakes are manual, no hydroboost or vaccum booster. I also have a 245 tire up front and a 285 tire out back. Tires are 200 tread wear. I can't lock the tires up, but it stops really well. It is a combination of the "sticky" tires and not having some type of power assist? Or is there an underlying problem I'm not aware of?