******,
That's the exact symptoms my truck had when the vacuum pump went out. I found a vacuum test online somewhere....Here's what I did:
Pulled a plug from the vacuum manifold on the driver's fender.
Plumbed my vacuum gauge into that port.
Fired up the truck. Vacuum, about 19" (should be 21" if I remember right).
Went into the cab, hit the brakes twice. Went back out and looked at the gauge. It was down to 5" or so, took it almost a minute to get back to 19".
Turned the truck off, it held the 19" for 15 minutes (that's all I left the gauge plugged in for) so I knew it wasn't a leak.
Bought a brand new vacuum pump from Napa ($130ish), pulley remover from HoboFreight for $10ish and went to town. Found out that the pulley on there was bent, got a new one from Ford ($30ish), reassembled and back in business.
Since then I've heard from a few folk who never mess with the pulley. Too easy to bend and they just replace them when they do the pump. My local International parts shop had the pump with pulley for about $175 if I remember correctly. Next time I'll go that route.
Mike