Can't get a firm pedal

[MichiganLarry]

After our 4x4 conversion the last issue is the brakes. The brake light on the dash has been on since we swapped the axles and bled the brakes. i'm thinking that the low fluid switch in the master is bad. That wouldn't be so bad but the pedal is quit soft and the it doesn't seem like the fronts are working up to snuff. Over the weekend I rebled the brakes and the pedal seemed ok with the motor off. When I started the truck the pedal went down a LOT further than with the motor off. I took the truck out on the gravel road in front of my house and tested. Forward direction hard breaking locks the rears up. If I backed up and hit em hard the fronts would lock but not as easily as I would have expected. I know there is a pressure differential swich somewhere in the system that is supposed to center after the brakes are bled and you push hard on the pedal. Is is possible this switch is stuck? Where is it located and can I physically center it? I was going to replace the front rotors and pads tonight but that is going to be around $100 in parts. Any ideas on what else I can check?

Thanks

Forgot to mention. Axles are 88 4x4 with 10" brakes in back. Master is 89 2wd had 9" brakes in back.