stoplight switches on m275 tractors

hi guys,
i replaced my rear lights, on my m275, i found out that my hydraulic stop switch (at the end of the long style airpack) is not working. i removed the connector and measured between the 2 contactpoints, 25 volt is present, i tried to measure between the 2 tips on the switch itself when pressing the brake pedal it should close the circuit (let the voltage go trough)...... it didnt......so its broken, however my stoplights are still working , when looking to the wiring schematic there is also an air activated stopswitch, and most likely this one is working . My question is do these 2 brake light switches work together ? (the air activated one only works offcourse with air in the tanks) thougth that one was only for the trailer ? why there are 2 switches on tractors? you dont drive the truck without air anyway...? or maybe a safety feature ? thanks

There are two stop switches so that if you apply the trailer brakes using the trolley bar ( brake control on steering column), the brake lights will come on.

NDT said:

There are two stop switches so that if you apply the trailer brakes using the trolley bar ( brake control on steering column), the brake lights will come on.

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ooohhhhwww yes, i didn't even think about that lever on the steering column....!!! Thanks!!! so the lever goes via the switch, air controlled, that's under the gladhand tower behind the cab together with the brake pedal, and the hydraulic switch is only activated by the brake pedal.

Johan, the hydraulic pressure switch is regarded by many as a weak link in the hydraulic brake circuit. You may want to consider putting in a second air pressure operated switch and replacing the hydraulic one with a pipe plug.

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cattlerepairman said:

Johan, the hydraulic pressure switch is regarded by many as a weak link in the hydraulic brake circuit. You may want to consider putting in a second air pressure operated switch and replacing the hydraulic one with a pipe plug.

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will do that but for now i have an other one here, so i will install this one and do the modification later. however i still dont know if the entire brake system shoud be bled or just the airpack alone after changing

I've had some luck getting the hydraulic switches working again by cleaning them out with brake cleaner and compressed air. Napa sells a universal switch that works good but uses screw terminals.

I think the NAPA/aftermarket switches are intended for lower max hydraulic pressures and have been known to fracture; the stock switches on the other hand seem to do that as well. I remember reading a thread about this a few years back. The military started changing them out for air side switches, certainly for a reason.

Ok..found it: https://www.steelsoldiers.com/threads/sorry-one-millionth-brake-light-question.30778/

For use in the Deuce hydraulic system the switch must be rated for 3000 psi...the NAPA switch is rated for half that.