4.0 Liter Vacuum Pressure?

Hey guys, I just had a quick question, I have a set of air horns that I took off of a Chevy truck with a 350. Now these air horns run off of the engine vacuum, it has a small compressor that uses the vacuum to pump air into a small reserve tank. I have a 97 Grand Cherokee with a 4.0, and when I hook the vacuum line from the air horns to the Jeep it doesn't seem to be putting out a whole lot of air, it's not even enough to get the horns to make any noise at all. I was just wondering if it was because the vacuum pressure on the 4.0 is a lot less then the 350? The 350 doesn't run so I can't test out the horns on that. I am going to take the pump apart tomorrow and take a look to see if I can spot any problems. Do you guys know of any place that sells these vacuum air compressors? And if I can't get this pump to work do you think its worth it to try and get a new pump just like this one or should I try an electric compressor?

Thanks,
Josh

Well, I went out today, took the whole pump apart (its really neat how the thing works off of vacuum), used some WD40 and got everything moving nice. Hooked it up to my Jeep and it works like a charm, it actually makes too much pressure, I let it go for a while and tried the horn and the solenoid wouldn't open! My buddy's got a solenoid hes going to let me try so if that works I'll be good to go.