Hi Tim, Looks like Scott and Dale are right on the money.
I would try the back off route first.

I had a similar problem a few years back with a VW Bug. One of the flex pipes was working like a non-return valve.
As Scott said watch out you dont 'cook' the linings or drums!
Good luck.

Jock

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.