Key won't turn in my 99 Plymouth Voyager

The brake fluid would have nothing to do with the ignition switch. As for the switch locked, try moving the steering wheel up and down if it is adjustable. And try turning or jerking the wheel hard left or right while trying to turn the key. At 112,000 the ignition tumblers or the key could be worn down! The brake master cylinder is on the drivers side at the top of the engine compartment. The fluid is plastic lite tan in color and has a removeable cap on top at the front.
If you wipe the plastic container off with a paper towel you should be able to see the level of the fluid. It is below the seam in the middle of the container it's too low. Fill it only half way between the seam and the top. If that don't work, take it back to whom ever worked on it and make them fix it. Provided you can get it to start!

Any DOT 3 break fluid will work