> > floating. However, there is supposed to be an internal 
> > pull-up resistor in the Pic itself.
> 
> Sure? Datasheet page nr?

I'm not a 508 user and can't confirm this from practical
experience but according to page 9, DS40139A, 12C5XX
data sheet -

When pin 4 (GP3 /MCLR Vpp) is configured as /MCLR the
weak pull-up is always on

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