The ByPass Diner opened in Harrisburg in 1939. It was a converted Brill trolley, formerly Hershey Transit No. 8. It was purchased from Brill for $5500 by David L. Cronin and H.P. Collins. Was the diner wing of Brill ever converting old trolleys, or did they simply broker the sale? I’ve never heard of them doing either. The old trolley was replaced in 1953 by a shiny new DeRaffele Diner, placed slightly differently on the lot (1933 Herr St, whereas the trolley was at 1951 Herr- the difference can be seen in the photos). That diner still operates as the American Dream Diner.
Diner Hunter 
Well, Historical Society of Pennsylvania has a lot of the Brill records up to 1930, though I don’t know if that includes their diner ops–most of the things HSP mentions are from the rail side.
Just in case: http://www2.hsp.org/collections/manuscripts/b/brill1556.htm#in-depth
Around 1926 Brill was acquired by American Car & Foundry, and their historical collection is now part of the John W. Barriger III National Railroad Library at University of Missouri – St. Louis.
http://www.umsl.edu/mercantile/barriger/index.html
Also, you might try throwing a Hail Mary by contacting ACF Industries directly, and asking their HQ if they can refer you to an in-house historian or expert they’ve worked with who can help you.
http://new.acfindustries.com/contact-acf/
Hope this helps!
Also, you might look into great-great-great-granddaughter of the company founder Debra Brill’s “History of the J. G. Brill Company,” published by Indiana University Press.
http://books.google.com/books/about/History_of_the_J_G_Brill_Company.html?id=eqKKrMi3FIIC