Soft brakes when cold

[Snowturtle]

My 2003 Yukon XL has soft brakes when it's cold. I get a humming from the firewall area for a few seconds when I release the brake pedal but can't get up front quickly enough to see where it comes from. The problem goes away once the car is warmed up and driven for a while. There are so many parts to the power brakes/ABS system that I don't want to just start throwing parts at it. Any ideas?
Thanks.

 

[Snowturtle]

No thoughts what pump might be running up front? Is there a unit separate from the master cylider that controls the ABS function?

 

[oh1yukonslt]

No thoughts what pump might be running up front? Is there a unit separate from the master cylider that controls the ABS function?

I'm not sure but how does the fluid look? Maybe a brake bleed/flush could work and solve the problem.

 

[Snowturtle]

Well, I checked the fluid and bled the brakes a little but didn't find any air. The fluid looks like clean brake fluid to me. I can't figure how a problem caused by air in the system would go away once the humming pump warms up. Where is there a pump in the system? I see a booster and a master cylinder. There is some other black cylindrical unit hanging off of the booster and hooked up to vacuum lines, what's that? I guess I need to buy a manual...