If the vacuum pulls it down but not up chances are it is the upper seal on the actuator, if it was the diaphragm it wouldn't go up or down. You can buy the upper seal separately. If it is a marginal issue sometimes some thick grease between the seal and the vertical shaft and the grommet area may be all it needs. The repro upper seals are quite failure prone though.
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My Break Away Squad
1969 Fbird (base OHC, 350, 400HO & Sprint Cvt’s - 400HO & TA ht’s)
1969 Custom S 2dr ht
1969 LeMans (2dr & 4dr ht & Cvt)
1969 LeMans Safari wagon
1969 GTO (2 Cvt, 2 ht & Judge ht)
1969 Cat (3 Cvt’s & a 2dr ht)
1969 Ventura wagon 2 seat
1969 Exec 4dr post
1969 2 Bonnie Cvt's & 2 wagons
1969 Bonnie Brougham (4dr ht & Cvt)
1969 Grand Prix 2 SJ’s
1969 2+2 2dr ht (Canadian)
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