yep you got a vacuum leak. Were you working on your truck recently? Because you might have pulled the vacuum tube from the coupling on the firewall. My guess is that you have a crack in the tube from the vacuum pump to the splitter on the firewall or one of those rubber coupling things is shot.
Check the vacuum line under the dash on the passenger side. The supply line comes from the vacuum pump, goes across the firewall to the passenger side, then goes in, then back across under the dash to the connector behind the ashtray. My truck did the same thing, and the problem was that the vacuum line was kinked where it came in thru the firewall. Gotta pull the line out to find it. I fixed the line and ran it back in thru another hole on the driver's side of the firewall, no need to pull the entire dash or go all the way back across to reconnect it.
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