The discharge air from a wet pump contains an oil mist, so these pumps require an oil separator in order to return most of the oil to the engine sump. Consequently, wet pumps aren't great for aircraft with deice boots, because the oil can cause the rubber to deteriorate. In our view, a little oil on the belly is a small price to pay for a vacuum pump that doesn't fail suddenly and without warning. Pressure systems use precisely the same dry air pumps and gyro instruments as vacuum systems do. The filters, regulator, and cockpit gauge are different.
Pressure systems suffer from exactly the same problems as vacuum systems do.
The dry pumps in pressure systems are equally vulnerable to contamination, overstress, and faulty installation. And the troubleshooting techniques and equipment are essentially the same. If one pump fails, the probability of the other pump failing shortly thereafter is decidedly non- trivial. These are excellent systems, and have the advantage that the standby air pump runs only when needed. Such systems are rather pricey, however.