I have a 1988 chevy 1500 pickup that keeps stalling out

When you do that your taking all the vacuum away from the engine. One cause is a lot of miles on the motor, the motor may not have the vacuum it use to. My engine did this also, and was a lot better when I put a new motor in. I notice when I brake hard now, and like go to pump the pedal the brakes are harder to push as the motor is now taking the vacuum. Otherwise it all works fine. I was thinking of adding another vacuum canister in line so there would be a build up of extra volume of vacuum on mine, if that makes any sense.