That's way low. It should be high 30s at idle and 40 or so while driving. It could be a clogged filter, it could be the pump or it could be a vac leak.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.