I agree with Steve... Something is not right.
If your car typically generates vacuum readings high enough for the brakes to work (say 16 in hg+), than I would think you need additional storage, not a pump. Pumps are for setups that cannot generate a high enough vacuum signal to begin with, as in radical cams (with more Overlap).
The vacuum system should be sealed well enough to provide strong brakes using reserve vacuum and not rely on a constantly high manifold vacuum signal to get the job done. If your running high boost pressures, consider clamping every vacuum line you have!
Master Power Brakes, which does offer a conversion kit for our turbo cars (About $500) as well as optional vacuum pumps, recommends running rather long vac lines to act as a reserve reservoir (Vacuum storage). Taking this line of thought a little further (and for piece of mind) I would think plumbing in a dedicated vacuum reservoir before the brake booster would be a nice inexpensive way to increase capacity of the braking system in 0 vacuum, boosted, or even slight vac leak conditions, as long as the check valve is moved ahead of the reservoir and that it seals 100%.
If you think about it, we already have a vacuum reservoir and check valve system in our cars for the ventilation system. If this system is working, even with the car shut down, you can get the vents to cycle at least once... Should be true for vacuum "Conversion" brakes as well!
But all of this is useless if your brake booster diaphragm or the check valve leaks. If you shut your car off in the driveway and wait a moment or two, you should still have at least one power assisted brake press. If not, I would suspect you either have a leak, or the check valve/grommet needs replacing.
Just to let the other members know: I have just converted with used components, and I can boost to 10psi and "Panic Stop", including locking up the wheels if I want, without issue. If you can't, something is wrong with the parts or the install, not the concept.
Vacuum brakes are reliable, and have proven this for years... they work in momentary 0 vacuum conditions (Panic stops) without issues. The big difference is that in non-boosted applications, the brakes will still work reasonably well in spite of small leaks. In our cars, it is just a bit more critical to seal these systems well and prevent boost from canceling out the vacuum stored in the brake booster.
I hope this helps a bit! If you still need a vacuum pump:
http://mpbrakes.com/mp27.htm