1989 buick electra park avenue when pump copmes on for vacuum system it runs constantly and I can hear air by both back tires. It sounds like a bad tire leak it is so loud. What is causing this and what is it pumping air to back there?

You have a 29 year old car.....the lines to the air shocks/struts are plastic, I suspect they have been damaged or just failed because of age.....you will probably need to replace the plastic lines

ty. its my sons car. I will get under there and look now that I know it is lines going to the shocks.