sounds like a vacumn hose has become disconnected someplace..
have a look in the engine bay...there should be a black hose from the firewall going to a vacumn outlet off the inlet manifold..
they pulled out the dash to replace the heater core? if they did a vacumn hose might have not been put back on 100% or fallen off...
First Ever Car - '84 XF Fairmont Ghia....Big Luxo Tank, Just Gotta Love that Velour!
Previous Ride- '87 XF S Pak...Now Retired with 305,000 on the clock!
Previous Ride-'99 AU Falcon S,Got sick of the 6...Need a V8!
5/1994 ED Futura Classic 5.0 in Polyneisan Green - One of the last ever built