The front and rear reservoirs in the master installed on my car right now are identical in size, if Im answering this correctly. Im assuming your "changing from drums to disc"? I had a "larger" master in the car last year but it didnt look right to me in an A body, but worked just fine. Bought it new from MPB. Swapped in an OEM from local auto parts store. You may be able to look up on the MPB website to see if they specify front disc or drum with a different p/n.