1. The fact that the tractor sometimes will not go at all when in gear indicates the sprague on the low side of the TA is worn and slipping. The clunk you feel when it does then pull is the sprague clutch locking up. A properly working sprague will always lock up and pull on the low side even if there is no hydraulic pressure from the MCV system.
2. More importantly you have a problem with the MCV hydraulic system. Quite possibly this is may be a failed o-ring seal between the clutch housing and the rear frame allowing air to enter the system between the filter and the MCV pump. The way to check for this problem is to make sure that the oil level is on the full mark on the transmission dip stick and then add another five gallons of addition oil. This will insure that the o-ring is now below the oil level and any leakage will only draw in oil and not air. If this cures the problems, just run the tractor with the oil over filled to this level. It will not cause any other problems.
The reason that maintaining good oil pressure in the MCV system is so important is that in addition to providing oil to operate the steering and brakes, excess oil is used to lubricate the differential. Problems with the MCV system can lead to differential failures.
The MCV pump is a nine gallon per minute (at rated speed) pump. The oil first goes to a priority flow divider where 3 GPM is diverted to the steering. Any oil above the 3 GPM is sent to the oil cooler in front of the radiator and then routed back to lubricate the differential. If the MCV pump system cannot provide in excess of three gallons per minute there is no flow to the oil cooler and differential.
The 3 GPM flow is sent first to the steering system. This system is protected by the steering relief valve located in the bottom of the MCV. Oil returning from the steering or bypassed by the steering relief valve is then available to operate the brakes through the brake check valve located on top of the MCV. Early tractors like your 806 have an open center brake valve which passes one GPM through the valve and returns it to the reservoir. This flow goes to the reservoir whether or not the brakes are being applied. Later tractors have a closed center valve that passes this one GPM only when the brakes are applied. The pressure to the brakes is regulated to about 235 -285 PSI by the pressure regulator valve in the MCV. After the brakes this oil flow is also directed to the TA control spool valve and the TA pressure dump valve. The spool valve sends oil pressure to either the high side clutch or the low side clutch in the TA assembly. The TA dump valve, when the clutch is depressed, returns this oil to the reservoir releasing both TA clutch packs. The TA assembly is a closed center system so unless there is a problem there is still 2 GPM flow available to go to the TA lube system. The TA lube oil is regulated to about 20 PSI. The lube pressure sending unit monitors this pressure and should turn the oil pressure warning light on if the pressure falls below 3 PSI. This is normal when the main clutch is released.
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