[Arran]

 Since the set used to play years ago before your father moved that's an indication that the tubes are more then likely still functional, it isn't too often that they will go bad from just sitting there. Capacitors and resistors, on the other hand, do go bad with age, or at least capacitors and resistors from the 1930s, 40s, and 50s, the resistors tend to drift upward in value, and the capacitors become leaky or open with age, the weak volume is a good indication that this has happened.
 There is one other reason why you should refrain from powering up this set until it has been gone through, and that is the rubber covered wiring that Philco, and some other makes, used in the period between 1939-42, the insulation can often deteriorate and cause short circuits to the chassis. Sometimes you can be lucky and find that it is still sound and plyable, other times it will literally crumble if you touch it, or become gooey and drip off the wires.
  One more thing, if you are thinking of restoring this set to keep as an heirloom or for nostalgia reasons it makes perfect sense, but these are among probably the least fruitful radios to restore and flip for a profit unless you are really good at working on them. That being said these Philcos are decent performers, and style wise they are not bad looking with a fairly good quality cabinets.
Regards
Arran