Air dryer is "common" to front and rear.
Ok last weekend I sprayed soap and water on the hoses near the airbags and couldn't see any leaks (not much hoses visible). With the batt disconnected the whole truck goes down so there has to be something in common. The front was slightly lower than the back. Any ideas guys. Thanks
This is a tough one. It is not theoretically possible for all four corners to drop about the same amount with the battery disconnected or F26E removed, yet it does. The fact that with the battery connected and power to the air system, it drops about double, that makes sense and indicates the auto level circuit is alive and well and doing its thing even if it is not what you desire.
In theory, when the system is depowered, the front and rear block valves fail closed and hence the air within the respective air springs is supposed to remain trapped within assuming no local leaks.
About all I can think is that both block valves are not sealing perfectly when depowered, (possible with age), and that there is a mechanical leak somewhere within the air compressor. Once air gets past each of the front and rear block valves, only the air compressor bits would hold that air within the system.
It is fairly easy to get at the compressor, (easier than the front block valve), to do a soap test so that is where I would start. The compressor covers and compressor probably needs cleaning anyway and there are lots of lines and connections within to test as well.
It is possible the air dryer plastic has developed a crack or the connecting seal is leaking. That is what I would check first and it would be a nice fault as replacing the air dryer is almost good routine maintenance and relatively easy to do.
The next logical leakage point would be internally - the mechanical relief valve, but that is hard to detect; next would be something called a delivery valve seal. The valve seal is possible as in early units it was the subject of a repair call up. Neither of these areas lend themselves to a soap test however as if air is leaking, it will just escape thru the exhaust lines as it is supposed to do or back thru the air inlet line - neither is easy to detect.