Is This Even Possible? Vaccum Related

[TX '90 ZR1]

I started up the car (1990) this morning to do a fuel pressure test. (I will share my results of this in a separate thread).
At one point I had a backfire. My first thought was "I bet that blew a hose off!"
Shut it down and let it sit a minute. Turned the key back on and sure enough the vaccum pump ran & ran. Walked away for a few minutes a little disgusted. Have plans to drive the car in a couple of days and didn't have time to tear into it before then.
Decided I might as well check it out. Checked the MAP sensor hose and the fuel evap hose thinking I might get lucky. Both obviously connected. OK. I turned the key back on thinking I might be able to hear the leak and have an idea of what I was getting into. By the time I could get from the drivers door to the engine the vaccum pump cut off. What the hell? Guess the pump is out now!
Key back off. Disconnected the hose from pump and there was vaccum in the system??? Turn key on. Pump runs. Put finger over hose and the pump goes off. Pulled 12 in.Hg on the vaccum system. Took it 15 minutes to leak down to 9 in.Hg. Obviously not leaking much.
Drove car. Secondaries actuate. Idle normal. Runs like it should.
I really didn't dream this up. It really happened. Anybody ever hear of something like this before?