I am not sure without my book, but since there were only 30 odd of these compared out of the PRR's entire heavyweight fleet at its max at around 150 before WWII, then with subsequent replacement by lightweight diners, it is possible but not likely. I would have to look at my book to see how many survived compared to other class that were larger. The D78bR was a smaller class with (I think) the D70 being the largest class.
EDIT: Poked around Jerry's site and found this (this is for 1954 since this is Jerry's modeling year):
18 D78b (I suspect by 1954 the PRR dropped the R since everything was air conditioned)
123 Total Heavyweight Diners
This means that the D78b fleet was 14.6% of the total fleet, but the only class larger than it is D70a with 30 and the D78d also has 18. So I guess it is possible and unlikely that they kept the same heavyweight diner on The Admiral, so I would say that a D78b made it on every so often.