Re: trim tilt fluid johnson 150
when the system is down, all of the piston shafts are inside of the cylinder housings. An amount of oil equal to the shafts gets pushed back into the reservoir. The reservoir is designed to be airtight. The trapped air compresses to compensate for piston shaft movement.
when the system is lowered, the reservoir is pressurized. When the system is raised, the reservoir is not pressurized.
when the system is fully raised, all of the piston shafts are outside and the cyclinders are 100% oil filled. The air above the reservoir is decompressed and the fluid level is to the bottom of the filler plug.
If there is air in the system it will get pushed back to the reservoir and float to the surface. It is possible for a very small amount of air to become trapped under the piston and create a few thousandths inch of "bounce". In the factory, this air is eliminated. In the field, it is ignored. All other air gets forced back into the reservoir as the system is cycled up and down. Once the air reaches the reservoir it floats to the surface. As such, trim systems are self-bleeding. You simply fill while the system is fully raised, put cap back on, cycle a few times, and recheck. Some systems have a very fine nylon mesh debris filter. The filters can hold air bubbles. When working with such a system it is good to go have a beer before rechecking fluid levels.
