Why is it better? It doesn't WORK better, assuming that the Powermaster works. In fact the Powermaster, if it works, is probably better overall. If it works. The advantages of vacuum brakes are simplicity, reliability, and price. The Powermaster has an electric motor that runs a pump to compress the gas in the accumulator, which is what gives you power brakes. It has a pressure-sensing switch that makes the motor and pump run when the pressure in the accumulator drops to a certain point. This is a complicated system and is prone to failure.
1. The electric motor can, and does, fail. Vacuum brakes don't have one.
2. The pump can fail. Vacuum brakes don't have one.
3. The pressure-sensing switch can fail, causing the motor and pump to run until they also fail or the fuse just pops. Vacuum brakes don't have this switch either.
4. The accumulator can leak, which drops pressure, which causes the motor and pump to run until they fail or pop the fuse. No accumulator on vacuum brakes.
5. Oh yeah, and the master cylinder (usually seals on the piston inside the cylinder) can fail--and this can happen with either kind, Powermaster or vacuum.
FIVE POSSIBLE POINTS OF FAILURE. Four of which are pretty common with Powermasters (1-4).
1. The booster (vacuum storage device) on vacuum brakes can fail (although I've never had it happen in over 30 years of car ownership).
2. The check valve on the booster (keeps vacuum from leaking out of the booster) can fail. This is more common than a booster failing. Keep a check valve in your car. About $2.
3. The master cylinder, as above, can fail. A flaw all brake systems have.
THREE POINTS OF FAILURE. And they are relatively rare and relatively cheap to fix...
Keith