pedal is soft at the bottom

Now you're cooking with gas!
I don't think your earlier all-drum system came with a P-valve, but with a Safety switch only. If you are running 275s or bigger out back, and 235s or less up front, I think you will be fine without a P-valve. However, only testing will prove it. I gutted my P-valve shortly after I wore the 245s out,back there; probably 2001 or so.