I have an 04 Dually. The power pump has been replaced twice. GM has a recall on the booster and it has been replaced. On deceleration towing after it's hot, the booster howls and on occassions, the brakes fade. It must be the booster because the tone of the noise changes based on the amount of pressure applied to braking.

It's been doing this since it was new but only after 40K miles are the brakes starting to fade when this happens?

Is there a fix?