Which vacuum pump do you have? The early diaphram types require frequent maintenance, where the newer rotary vane types will go years and years. My '86 has the rotary vane pump, produces 21 inches of vacuum steadily.
Are 1.6 vacuum pumps adjustable (vacuum settings). I switched to Scirocco 10,1 disc, and it feels like manual (non power assit). Had the pump rebuilt. What vac. should it pull @ what rpm?
if you switched to bigger brakes, and used the original master cylinder, it MIGHT cause issues.
quick check as to if the booster is getting vacuum, fire it up, take the vacuum line off the brake booster, and check that it has decent vacuum. put your finger over it, see if it sucks pretty good. its not a very scientific way to test it, but it doesnt need a specific vacuum either..
did you check either of the check valves in the setup? assuming you have a diaphragm style vac pump?