Hi Milt,
They are both for the left normal brake system, one for the inboard wheel and one for the outboard. There are another pair on the other side of the aft wall for the right normal brakes and also one each for the left & right alternate brakes.
Yes the white balls are reset levers (see below).
There are no return lines. The fluid goes to & from the brakes along the same line. The fuses are the last things in the whole system before the brakes, (except for the shuttle valves to use the alternate braking system) so even if a wheel falls off, the fuses will close, no further hydraulic fluid will be lost and no air will be sucked back into the system. Brilliant.
Below is an extract from the 737NG AMM
S & L
“Brake hydraulic fuses prevent hydraulic fluid loss if there is an external leak downstream of the fuses.
Functional Description
During normal operation, the piston and spring are at the relaxed position. This lets fluid pressure go through the metering slot and over the bypass valve normally. When the pressure differential across the fuse starts to decrease below normal, the piston starts to compress the spring.
If 60 to 95 cubic inches (0.9-1.5 litres) of fluid goes through the hydraulic fuse metering slot and bypass valve, the piston compresses the spring until the fuse
is closed. When the pressure differential across the hydraulic fuse decreases between 0 and 30 psi, the fuse resets. This lets the spring push the piston to the normal position.
A reset lever permits manual reset of the hydraulic fuse. To reset the hydraulic fuse, move the reset lever in the direction shown on the placard near the fuse. This operates a bypass valve inside the hydraulic fuse that makes the pressure on each side of the fuse equal.
There are no visual indications of a closed fuse.”