Vacuum brake conversion won't hold boost at line

[V6POWER]

I did the vacuum conversion and went to the track today. I can't hold more than 3# boost at the line while brake torqueing. Is there a way I can improve on the brakes in order to be able to hold the 5-8# that I could hold with the powermaster or do I have to go back to the powermaster?

 

[Red Regal T]

Soft brake shoes, S-10 wheel cylinders, E-brake..........