Piston assembly Bleed port Spring Pressure regulator Slide assembly Port No. 1 brake pressure Port No. 3 system “A” pressure Spring-loaded check valve Brake metering valve shaft Piston Roller Crank arm Port No. 2 to return FWD UP Brake accumulator Nitrogen charging valve Accumulator pre-charge pressure gauge Brake pressure gauge and accumulator air charging valve System brake accumulator System A brake accumulator Figure 13-102.  The parking brake lever on a Boeing 737 center  pedestal throttle quadrant. Figure 13-100.  The power brake system on a Boeing 737.  Figure 13-101.  Emergency brake hydraulic fluid accumulators are  precharged with nitrogen to deliver brake fluid to the brakes in the  event normal and alternate hydraulic sources fail. 13-60 Parking brake lever