Peter,
I have checked the US censuses for Percival Drayton, and believe I have found him in the 1850 and 1860 ones.
For 1850 he is listed in the Spruce Ward of Philadelphia, living with his step brothers William and Henry Drayton. For 1860 He is listed (mistakenly it appears as Peter Drayton) in the 7th Ward of Philadelphia, living with his step-mother Maria Drayton and his step-brother Henry Drayton. It may be the same dwelling for both, just a change of nomenclature for the ward.
As you may know from his service record at the time (1860) Percival was ordnance officer of the Philadelphia Navy Yard.
If you would like I could e-mail you copies of the census pages.
Regards,
Ralph Osgood