B737 Brakes Hyd return

[NSEU]

"There must be a blocking valve on the A return as well!!"

I share your confusion, LEM, but, as far as I can see, the blocking valve, if you look at the brake system schematic, is only required when there is no continuous pressure on the brake pedals or the hydraulic systems have been turned off.

When the brake pedals are depressed as part of normal park brake application, they are held down by a mechanical latch. Therefore, you have continuous pressure on the brake pedals. If you then have hydraulic pressure, the blocking valve is not required....it's just like normal pedal braking. If you remove pressure from the pedals or remove hydraulic power, then you have no braking.

Let me expand/paraphrase....

When the brake pedals are pushed and held by the park brake mechanical latch and you have pressure from system A (B off), then you have braking (the latch simulates your feet on the pedals). However, if you switch off System A, you no longer have any brake pressure and therefore no braking. This is where the brake accumulator comes into play (on system B). Assuming there is pressure in the accumulator, then the shuttle valves will position themselves to allow pressure from the accumulator to reach the brakes (pressure removal from System A will allow the shuttle valves to move to System B/the accumulator circuit). The normal park brake valve on System B can now come into play (if I interpret the diagrams correctly).

The only problem I can see is if there is a loss of fluid or pressure in the accumulator braking circuit.

I stand to be corrected.

Rgds.
NSEU