Air brakes · layered visual atlas

One brake pedal operates the front and rear sections of a dual foot valve at the same time. SECONDARY supplies front service braking; PRIMARY supplies rear service braking. In a documented TP-5 arrangement, the command selection uses front and rear pedal signals plus the hand signal. The main layers group the pedal’s two signals as one PEDAL COMMAND; that is a functional simplification, not a new merged reservoir or single physical port.

The compact BRAKE PEDAL block shows two separate, equally spaced controlled air passages. These colored passages are dashed when service is not commanded and both become solid on Pedal applied. Tank-to-valve supply pipes remain solid: inlet pressure is still available. Dashed colored passages indicate the uncommanded valve state, not mechanical linkage or an open supply bypass. The hand-lever drawing is an illustration, not another air connection.

The tractor rear service relay follows a pedal command and supplies air to apply the rear SERVICE section. It is not the trailer spring-brake control valve. We have hidden its box, not removed its function. The primary reservoir does not directly apply the rear brake without pedal-controlled valves.

GREEN secondary/front and PURPLE primary/rear are circuit roles—not universal tank locations or paint colors. We call the first collecting reservoir SUPPLY TANK; wet tank is another name for it in this layout. The grey dotted connection to the governor carries pressure for sensing; it is not mechanical linkage. Governor placement on the compressor is representative; remote mounting exists. The charging check valves and supply selector are different functions.

SR-5 trailer spring behavior is a documented example. Check stages, exact hand-valve source, rear relay, tractor spring-control hardware, ABS, anti-compounding and many exhaust passages are simplified or hidden. Both service controls released means service air exhausts; rest state assumes sufficient air and parking controls released.

In 3A, command-selection detail is temporarily hidden: the shown blue route represents the pedal-controlled service command, not unrestricted reservoir supply. In 3B, the selector and independent hand input are added. Blue and red cross the tractor protection valve through separate internal passages; they do not mix.

The two separate dashed rear lines show which section applies mechanical force to the SAME wheel brake. They are not air inlets or two physical pushrods. Pedal applied makes only the SERVICE indicator solid; Parking brake applied makes only the SPRING indicator solid. Separate colored air inlets remain unchanged.

The trailer SPRING dashed mechanical-force indicator becomes solid only with Red pressure lost. Parking brake applied remains a tractor-only demonstration. Separate indicators represent action on shared wheel-brake hardware, not separate physical pushrods or air pipes.

The Parking brake applied button in layer 2 and the complete view isolates the tractor spring/parking function. The bold yellow path shows air exhausting from SPRING, not air being supplied to apply it. Grey reservoir supply is isolated from the exhaust. Actual vehicles may couple yellow and red parking/supply controls; those interlocks are not simulated here.

Service air APPLIES braking. Loss of spring holding air permits the spring to APPLY braking. No flow into a full reservoir is not pressure loss. Older trailers can provide emergency braking differently.

Iowa CDL manual, sections 5–6; TC-2 hand valve; double-check supply versus command; SR-5 spring control; TP-5 selection/protection. Not a repair manual. Never dismantle a spring chamber.