Mushy Brakes at Low Speeds

[davepl]

Normally your vacuum canister should hold enough vacuum for one or two brake applications such that you wouldn't notice the lower vacuum available at idle/low speed. But if the reservoir part has failed, you'll notice it. Check to see if it holds vacuum for a minute or two after shutting off the truck.

If its not a vacuum advance issue, have you touched the brakes? Could be air in the lines, because low vacuum usually is described as a harder pedal, or taking more force.