Power Disc Brakes not working on hard stop?

Hello, I wanted to see if you guys had any insight into my power disc brake operation. I've got a 67 Dart GT 440, 4spd with power disc front brakes and rear drums - stock components. I have a manual proportioning valve to limit the rear brakes. This setup is 10 + years old and under normal driving conditions works fine. However, when applying brakes in a hard stop it feels like the pedal hits a hard stop but the brakes don't feel fully engaged. No matter how hard I press I don't get anymore braking power. I'm wondering if this is a limit of the early stock disc brakes or if I have something else going on? It's been like this since I put the car together and I'm finally going through the car and tweaking the stuff I'd like to work better. The question is this normal and if not what can I try to correct?

Thanks in advance