Ronmar
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I don’t have any emergency brakes! You probably don’t have any either…
Emergency brakes: someone came up with a method to use the park brake springs with pedal input in the event you loose your primary/rear brake air. Its Completely automatic, no additional operator workload. its the whole reason to put in that inversion valve in the park circuit.
That inversion valve in the park air circuit compares primary air from the tank and secondary treadle air. It uses this info to control how park air from the dash control is applied to the anti-compound relay to compress the park springs/release the park brakes. Primary tank air should always be greater than secondary air from the pedal, this(and a small spring) holds the inversion valve spool at one end so park air from the dash control can flow right thru the middle of the inversion valve spool, to the anti-compound valve input to compress the springs.
But lets say you blow a hose and have lost primary air. When you step on the pedal, without primary air to keep the inversion spool at one end, the secondary treadle pressure on the other end forces the spool to shift against its small spring. When it shifts, it cuts off supplied park air thru the spool and vents the park air being fed to the anti-compound valve. As park air is vented, the anti-compound valve in turn vents air to the springs causing the springs to start to apply braking force. When you let off the pedal, the spring in the inversion valve pushes the spool back, re-applying park air and releasing the brakes.
Without this, you only have the few pedal presses left in the secondary tank and the single can front axle brakes to safely slow and get this beast to the side of the road. Emergency braking should give you ~50% of the normal braking force to the rear axle.
There is an “emergency” lamp in the dash, but our low bidder friends at S&S neglected to install a pressure switch to the output side of the inversion valve or wire it to that emer light to indicate when the park air is being vented by the inversion valve and this feature is being activated.
So thats how it is supposed to work. Now here is why it wont ever work on my truck(and maybe yours also).
I have written about and done some videos about how the anti-compound valve should work and how to test it. The anti-compound is a relay valve that has 2 inputs thru a built-in 2way check valve. One input is park air to release the springs, the other input is treadle air to also release the springs to prevent damage if you step on the pedal without releasing the park brakes(compound forces).
in looking at the diagram today researching @GeneralDisorder s dryer issue, What finally dawned on my pea brain this morning was the fact that my anti-compound is not plumbed like the diagram in the schematic. In the schematic that treadle source to the anti-compound valve is supposed to be from the Primary side of the pedal, so if I lost primary air, that input would also be missing. But in reality on my truck it is connected to the secondary side of the treadle, in fact it is “T”d off the very line that feeds the inversion valve For emergency braking
So on my truck if I loose primary air, when I step on the pedal, secondary treadle air will be shifting the spool of the inversion valve and venting park air to try and engage the park springs at the anti-compound valve. But at the same time, that very same secondary treadle air will also be supplying the anti-compound valve control input keeping park air applied to the springs and cancelling out emergency spring brake application.
With the line with red X’s connected, emergency braking will never work!
instead the line should be like the line I drew in blue coming from a primary treadle source.
So far the only place I see the actual connection specified is in the schematic diagram, and that shows primary treadle output feeding the anti-compound. The replacement procedure for those valves doesn’t even show the line functions or destinations, they simply say to mark everything well and copy the fitting orientations during replacement, and of course no procedure to actually confirm it is working
S&S strikes again, and it only took me how many years to notice it i must be getting old…
Emergency brakes: someone came up with a method to use the park brake springs with pedal input in the event you loose your primary/rear brake air. Its Completely automatic, no additional operator workload. its the whole reason to put in that inversion valve in the park circuit.
That inversion valve in the park air circuit compares primary air from the tank and secondary treadle air. It uses this info to control how park air from the dash control is applied to the anti-compound relay to compress the park springs/release the park brakes. Primary tank air should always be greater than secondary air from the pedal, this(and a small spring) holds the inversion valve spool at one end so park air from the dash control can flow right thru the middle of the inversion valve spool, to the anti-compound valve input to compress the springs.
But lets say you blow a hose and have lost primary air. When you step on the pedal, without primary air to keep the inversion spool at one end, the secondary treadle pressure on the other end forces the spool to shift against its small spring. When it shifts, it cuts off supplied park air thru the spool and vents the park air being fed to the anti-compound valve. As park air is vented, the anti-compound valve in turn vents air to the springs causing the springs to start to apply braking force. When you let off the pedal, the spring in the inversion valve pushes the spool back, re-applying park air and releasing the brakes.
Without this, you only have the few pedal presses left in the secondary tank and the single can front axle brakes to safely slow and get this beast to the side of the road. Emergency braking should give you ~50% of the normal braking force to the rear axle.
There is an “emergency” lamp in the dash, but our low bidder friends at S&S neglected to install a pressure switch to the output side of the inversion valve or wire it to that emer light to indicate when the park air is being vented by the inversion valve and this feature is being activated.
So thats how it is supposed to work. Now here is why it wont ever work on my truck(and maybe yours also).
I have written about and done some videos about how the anti-compound valve should work and how to test it. The anti-compound is a relay valve that has 2 inputs thru a built-in 2way check valve. One input is park air to release the springs, the other input is treadle air to also release the springs to prevent damage if you step on the pedal without releasing the park brakes(compound forces).
in looking at the diagram today researching @GeneralDisorder s dryer issue, What finally dawned on my pea brain this morning was the fact that my anti-compound is not plumbed like the diagram in the schematic. In the schematic that treadle source to the anti-compound valve is supposed to be from the Primary side of the pedal, so if I lost primary air, that input would also be missing. But in reality on my truck it is connected to the secondary side of the treadle, in fact it is “T”d off the very line that feeds the inversion valve For emergency braking
So on my truck if I loose primary air, when I step on the pedal, secondary treadle air will be shifting the spool of the inversion valve and venting park air to try and engage the park springs at the anti-compound valve. But at the same time, that very same secondary treadle air will also be supplying the anti-compound valve control input keeping park air applied to the springs and cancelling out emergency spring brake application.
With the line with red X’s connected, emergency braking will never work!
instead the line should be like the line I drew in blue coming from a primary treadle source.
So far the only place I see the actual connection specified is in the schematic diagram, and that shows primary treadle output feeding the anti-compound. The replacement procedure for those valves doesn’t even show the line functions or destinations, they simply say to mark everything well and copy the fitting orientations during replacement, and of course no procedure to actually confirm it is working
S&S strikes again, and it only took me how many years to notice it
i must be getting old…
