Well, I've had a good "dig around" in this one!
I went for Canadian units, hoping PCRR would be one of their provincial regiments, to no avail.
The fact that granddad was attached to a battalion indicates that the initials could be of a regiment but nothing comes up. Until I find an alternative I'll carry on looking for a Pioneer unit.
Pete, have you looked for granddad's medal card ? That should tell us what his parent regiment was, (a lot of men from the Labour Corps were attached to pioneer units).
Back to my spade !