Just some observations first:
"PORTC = 0 ' Clear PORTB", your comment says Clear PORTB yet your code states PORTC = 0
"MAIN:

GOTO MAIN" Add a pause in there; "MAIN: : PAUSE 10 : GOTO MAIN"

With that out of the way, what do you wish to look at for feedback? If you have an analog voltage, you can use an ADCIN 0, FB command followed with:
IF FB > X then
take action
endif
IF FB < X then
no action
endif

Let's start there before I type 3 pages and still not answer your question.

Mike