Brake booster ?

[7tee]

While stopped and at idle I noticed as I pushed on the brake and held it firmly, my rpm kept slowing down to the point I thought it was going to die until I released the brake and it came back. This is a manual tranny, stock motor with a very small cam. Initial timing is 12*, idle rpm 720 and vacuum is 14" with drum brakes all around. Was wondering if the booster was on the way out. There are no sounds coming from it like an air leak, just wondering...Ed