The reason PA's (especially Philly area) accent is so "hard" is b/c of the Welsh. While Boston (aka "Bah' sten") and New York (aka "New Yawk") have soft r's, Philly has HARD r's (as known by abyone who has been teased for saying "wood-er" instead of WAHter).

There are actually a lot of welsh settlements around here and a heavy Welsh influence on the way we speak.

All that being said, I (and everyone I've ever heard) pronounce the town name Bryn Mawr as "brin mar"