Broken vac line at fire wall - MAX AC only blows defrost but Vent works fine

[HarveyCat]

I have a broken VAC line at the firewall - maybe it is just cracked -- I think I need to change it -- but looked under glove box and need advice. MAX AC only blows defrost but the Vent setting works fine. The vent blows fine though the dash openings but the AC only blows to the windshield after a minute when set above the midway speed setting, but when it is set lower, if I am driving and the Expedition is bouncing around, then it will also switch right up to the windshield again. I have an Eddie Bauer Generation 2. (2002)

Any advice at all would be great!!!

Thanks