Firstly, I am not an experienced tractor-hydraulics mechanic. I’ve never bled a tractor’s hydraulics... Heck, I’ve never even serviced one.
But I do own one and I’ve operated hydraulic systems in various equipment from tractors to large airliners and supervised extensive maintenance operations on them. So here’s my opinion:
Trapped air can cause problems in hydraulics such as scoring, local over-heatings, lack of lubrication and cooling, and (rarely) “dieseling” such as when high pressure causes small explosions under high-pressure/heat conditions.
Hydraulic system maintenance (mx) can be very dangerous so think about what you are doing when you crack open a system.
There are basically 3 different types/ways air can be trapped in hydraulic systems, trapped air “bubbles”, entrained air, and dissolved air.
Dissolved air is the simplest problem.... It’s not a problem. Eventually it will coalesce out of the system when the hydraulic fluid eventually makes its way to the main reservoir and allowed to rest when the fluid cools.
Entrained air is usually in the form of “foam” or froth, and it will usually separate-out when the system is de-powered and allowed to rest for a period, and resort to trapped air capsules or pockets.
This last type of trapped air/capsules/pockets can sometimes be either moved to the main reservoir and allowed to return to atmosphere thru the vents when the system is shut down.... or can be “bled” via bleed valves or by using ad hoc techniques such as “cracking” open fittings while the system is at rest or at low activity. This last method can be DANGEROUS and can contaminate the area making clean-up hazardous. For this last reason, many hydraulics mfr’s and service-providers are reluctant to offer laymen instructions for bleeding the systems for liability reasons.
High pressure hydraulic systems can inject fluid into the skin, muscles, eyes, and penetrate organs and cause serious injury or even death. For that reason many entities simply refuse to issue instructions, considering it best to let the possibilities of degraded performance exist for privately-owned equipment.
The BEST method of avoiding the problem is to pre-fill components during installation/repair. When replacing a hose or line or cylinder... FILL it with fluid before installation and closing of the system. If that is done carefully then any remaining or captured air will be so small as to be of little influence and can be allowed to clear itself by ordinary usage.
If entrapped air pockets or large areas of air is suspected there are ways to get rid of it. One method, and likely the safest, is to. Position the equipment to allow the air to rise to the uppermost point near a fitting which can be cracked-open.... but this requires thoughtful consideration before doing it. Raising a FEL to it’s greatest height and opening a fitting up near the bucket..can actually CREATE or INCREASE the problem ... as the fitting is opened air can ENTER an unpressurized system as the fluid descends due to gravity. If the system is pressurized ... the danger previously mentioned of personal-injury and contamination exists. In such cases, it may actually be more beneficial to LOWER the FEL and crack open an UN-pressurized system at it’s highest point. Unfortunately this method allows the greatest spillage...and BE AWARE that cracking open ANY FITTING can and will suddenly release contained pressure and HEAVY EQUIPMENT/COMPONENTS CAN MOVE OR FALL ... a potentially dangerous result.
Sometimes living with the air and the associated risks is the better solution. After a while the air will either dissolve or work its way to the main reservoir and dissipate on its own... and that is why companies are reluctant to offer instructions which may place them into a liability/risk area.
Hope this helps.
