Those vacuum pumps last a very long time.
It is probably, leaking seals at the vacuum pump outlet, and bad hoses.
While measuring vacuum (I do it off the nipple on the large brake booster check valve when checking the pump's health) wiggle, pull, push on the vacuum pump outlet line. See if it doesn't have a sweet spot and go up to a good vacuum. By measuring at the nipple on the brake booster line, you eliminate all the system leak points except the large line, pump outlet, and brake booster itself. You have divorced all the small lines and actuators from the system.
20" is absolute minimum. A healthy leak tight system will have 24" at idle.
As DJR mentions, you may need to replace all of your small vacuum lines.
When testing the health of the system overall, I also measure at the vacuum source line on the N75. If it is good at the brake booster line, but bad at the N75, I isolate one component at a time, putting my gauge in place of the removed component and lines.
If you do use RTV or any other sealant on the vacuum pump outlet make sure that it is warm enough to cure, and you allow enough time for it to cure. 24 hours in summer weather for RTV is normal, since it is a thick layer. You do not want to suck any sealant into your vacuum pump, head, oil pump...