The power master is a different animal. Most mechanics scratch their heads. The reservoir itself is misunderstood.
The two tanks on the driver side are for front and back brake feeds. The passenger side of the reservoir is the power assist fluid. That side usually gets over filled by the mechanics but in your case I'm thinking you don't have enough. The pump is running non stop trying to charge the accumulator but is running dry resulting in the vibration your feeling in the pedal. If this is the case the passenger side of the reservoir is prolly dry or close to it.
With the key off press the brake pedal 10-15 times. Then check the brake fluid in the passenger side of the reservoir. It should be close to full. If not, fill it up and replace the lid. Turn the key on. The pump will cycle and start pulling the brake fluid out of the reservoir and into the accumulator. As the accumulator charges up the fluid level will go down to almost nothing in the reservoir. Right before it looks like it's going dry the pump should stop.
HTH
RL