An air pressure regulator includes an air input stage, an air output stage, and a transfer stage. The air input stage, air output stage, and transfer stage arranged substantially in-line, end to end along a longitudinal axis. The air input stage is attached to the transfer stage with first fasteners that simultaneously engage with the transfer stage and pressure fit a first pressure step-down piston against an input stage backpressure assembly, and the air output stage is attached to the transfer stage with second fasteners that simultaneously engage with the transfer stage and pressure fit the second pressure step-down piston against an output stage backpressure assembly.