Getting my 88 suburban Silverado going again

[Cursh]

Howdy yall.

I’ve owned this r20 454 suburban since 2008 but it was stolen at the end of 2019 and sat in my yard for over 3 years. I traced down the starting issues and have it turning itself on and moving forward and turning, but when I hit the brake pedal I hear a loud WOOSHing sound coming from under the dash.

Any idea what that is? The brakes work better than the sound led me to believe that they would, but I’m not trying to drive through the house or retaining wall to figure it out.

Any ideas?

What else should I sort out other than the brakes before I go for a slow neighborhood test drive?

Thanks for your help.

A little “before” and “after” shot because who doesn’t love pics?