the body 1 should be machined from brass with pressure area 60 for the tire pressure and pressure channels 61 to channel the pressure into the right areas . body 1 also has a battery cavity 62 and a test button cavity 63 . pressure area 60 , valve stem 24 , inlet adapter comprised of 23 a , 23 b , 23 c and test button cavity 63 are all connected through pressure channels 61 in the body 1 . the batteries 13 have a positive end 13 a in contact with the battery center pin 15 and a negative end 13 b completes a circuit and allows current to flow when a battery spring 16 is in contact with a battery end cap screw 14 to make the body 1 a negative conductor . the positive side is connected from the battery center pin 15 via the connector screw 37 to the connector strip 36 to the pressure chamber closing pin 10 to the pressure chamber contact spring 11 to the pressure chamber 9 which has the membrane 7 soldered to it . if the pressure in the pressure chamber 9 is greater than the pressure in the pressurized area 60 the membrane 7 will be activated and move enough to touch the support contact washer 4 which is in direct contact with the pc board complete with radio 3 and provide the positive power to the pc board complete with radio 3 . the negative side of the pc board complete with radio 3 will be in constant contact with the chamber threads 6 which are in direct contact with the negatively charged body 1 . the sensor / transmitting unit 100 also achieves its objects by having a mechanical sensor , membrane 7 , for the pressure drop . the membrane 7 on the pressure chamber 9 is a mechanical device and requires no electricity . when the reference pressure in the pressure chamber 9 exceeds the tire pressure by 8 psi the membrane 7 will move and contact the support contact washer 4 and the pcb circuitry will be energized . the monitoring is using no battery energy since the batteries are totally disconnected until the tire pressure has dropped 8 psi . this maximizes the battery life . instead of using the mechanical sensing system with a pressure chamber with a membrane , the sensing system can be replaced by mechanical spring loaded or an electronic sensing device after installing the sensor / transmitting unit 100 on its mounting bracket 27 and the flex hoses 39 are connected to the tire valve stems 24 the sensor / transmitting unit 100 can be calibrated . the calibration is done by turning the lens 2 counter - clockwise about one turn until it stops . the lens 2 turns the chamber threads 6 which in turn pushes the pressure chamber 9 towards the pressure chamber closing pin 10 which with the help of the o - ring for chamber pin 32 seal the pressure chamber 9 . the pressure chamber 9 is located inside the pressurized area 60 and will now therefore have trapped reference pressure 9 a which is the same pressure inside the pressure chamber 9 as inside the pressurized area 60 . the lens 2 then seals the pressure chamber 9 when it is rotated back to its original position . in a preferred embodiment , if the tire pressure drops 8 psi or more from being in direct connection with the pressurized area 60 through the pressure channels 61 the pressure in the pressurized area 60 will also drop the equal amount . the pressure chamber 9 will therefore have the 8 psi more pressure than the pressurized area 60 which will activate the membrane 7 which will touch the support contact washer 4 which completes the electrical circuit and activates electrical circuitry on the pc board complete with radio 3 which start flashing an led on the pc board complete with radio 3 and sending out an rf signal to be picked up by a receiver unit ( not shown ). to stop the pc board complete with radio 3 from flashing and transmitting rf signal the tire pressure has to be brought back to the original pressure where the reference pressure was set or the sensor / transmitting unit 100 needs to be re - calibrated . the rf signal can be pre - programmed to have three quick signals then a two second long pause , and then repeat the cycle to keep energy output from the batteries 13 efficient . the sensor / transmitting unit 100 also achieves its objects by being capable of being calibrated to any tire pressure range used on trucks or heavy vehicles . the system is based on the pressure differential between the pressure chamber 9 and the tire pressure and can therefore adapt to any pressure . the membrane 7 can be designed to move at pressures other than 8 psi , for example if a higher degree of control were wanted the membrane 7 could be designed to move at lower pressure differentials , even down to 1 psi or lower . a preferred embodiment of the sensor / transmitting unit 100 uses the same frequency and protocol for all units because service personnel could easily monitor vehicles as they returned to a maintenance or storage facility . the range of the sensor / transmitting unit 100 to the receiver is a distance of approximately 60 feet . however , it is possible to create sets of sensor / transmitting units 100 and receiver with different frequencies to reduce the possibility of triggering false readings from nearby vehicles . the sensor / transmitting units 100 is robust in air pressure measurement and is insensitive to changes in ambient air temperature because when ambient temperature drops both the reference pressure in the pressure chamber 9 and the tire pressure will drop equally . an additional benefit of the sensor / transmitting unit 10 is mounted on a bracket at the end of the rear axles for dual wheels or “ super singles .” this position allows easy accessibility so when filling tires with air personnel do not have to get on their knees . the functionality of the sensor / transmitting unit 100 and condition of the batteries 13 can easily be verified by depressing the test button pin 20 . test button pin 20 presses down the valve stem core 22 which will perform two tasks simultaneously . first , the o - ring for test button 30 located at the bottom of the valve stem core 22 will close the pressure channel 60 leading to the valve stem 24 where the pressure supply from the tire is provided to the pressurized area 60 . second , to relive the pressure in the pressurized area 60 , because the pressurized area 60 is now unpressurized , the pressure chamber 9 has more pressure than the surrounding area and the membrane 7 will be activated and give contact to the membrane 7 which will touch the support contact washer 4 which will activate the circuitry on the pc board complete with radio 3 which start flashing and sending out rf signal ( s ) to be picked up by a receiver unit 200 . when the test button pin 20 is released , the pressure will return to the pressurized area 60 and the membrane 7 will return to its standby position where there is no electrical contact and the flashing and sending out of rf signal will immediately stop . a receiver unit could be installed in the cab of a vehicle which has an audible signal which will beep when it receives the rf signal from the sensor / transmitting unit 100 . the audible signal could be turned on or off . the receiver could also have an led or other light indicator to turn on when it receives the rf signal from the sensor / transmitting unit 100 . the visible signal could be turned on or off . the receiver could be wired to receive power from the vehicle battery or have its own battery . a receiver could also be portable , for example , for service or maintenance personnel . a portable receiver could have rechargeable batteries . in an alternative preferred embodiment the body 101 is configured with additional features to allow the sharing of air between two linked tires but at the same time have a safety feature that allow air flow to be cut off automatically in the event of a catastrophic failure of one of the tires , even if the balancing valve would manually left open fig8 - 14 . there are two modes described , the first is where a balancing valve lever 142 is in the vertical position . when the balancing valve lever 142 is in the vertical position the balancing valve plunger 144 is in the open position that allows air to flow freely between air channel from ( 163 to 165 ) 168 . when the balancing valve lever 142 is in the horizontal position the balancing valve plunger 144 is in the closed position that stops air from flowing freely between air channels from ( 163 to 165 ) 168 . there is an automatic balancing valve plunger 147 with a spring 149 that has a preset tension and can be fine tuned by twisting the adjustment screw 151 so that for example the air pressure in one or both tires drops below 80 psi ( eighty pound per square inch ) the balancing valve plunger 147 is automatically closed , thus isolating the air between the tires . also , in the alternative preferred embodiment a inlet adapter 123 can be substituted for a more durable connection between air lines such as at flex hose 139 , compare to the inlet adapter 23 a , 23 b , 23 c in the first preferred embodiment disclosed above . it will be understood that various modifications can be made to the various embodiments of the present invention herein disclosed without departing from the spirit and scope thereof . for example , various devices are contemplated as well as various types of construction materials . also , various modifications may be made in the configuration of the parts and their interaction . therefore , the above description should not be construed as limiting the invention , but merely as an exemplification of preferred embodiments thereof . those of skill in the art will envision other modifications within the scope and spirit of the present invention as defined by the claims appended hereto .