Your 756 has two separate hydraulic systems that share the same filter. The rear (hitch) pump provides oil to the hitch and auxilary valves. That part is probably fine since you are not having problems with the hitch or remote hydraulic cylinders
The front (MCV) pump provides oil for the steering, brakes, TA application pressure, hydraulic seat, TA lube pressure, transmission oil cooler, transmission and differential lube.
Standard flow from the front pump is 9 GPM at rated speed. From the pump the goes first to the flow divider valve. There is an orfice that allows 3 GPM to pass through, all the other oil is diverted to the transmission oil cooler in front of the radiator. Return flow from the oil cooler goes to the rear (Range) transmision and differential to lube those parts.
The 3 GPM flow goes first to operate the steering. The steering pressure is regulated to about 1900 PSI by the steering relief valve. Return flow from the steering and steering by relief valve is then sent to the pressure regulator which controls the pressure at about 245 PSI. This oil is available for the brakes, seat, and TA application. Early brake valves were open center and used 1 GPM all the time. Later brake valves were closed center and used 1 GPM only while the brakes are being used. The TA and seat are always closed center type valves.
The oil is then directed to the TA lube circut where it is regulated to 18 - 20 PSI. This should allow 3 GPM (2 GPM with early style brake valve) for TA lube. Any oil in excess of the 18 - 20 PSI lube pressure is returned to the sump.
In your tractor what is happening is that when you turn and use the brakes, there is not enough oil flow to maintain the operating pressures on the TA clutch and so the high side clutch start to slip. The low side of the TA is designed to have most of the load carried by a sprague clutch. In your case this clutch is failing which is why the TA slips on the low side. If the low side cluch were holding you would feel the TA downshift to low instead of stopping when you turn and use the brakes.
The reason it is critical to have this problem repaired is that when you are having this problem, you do not have enough oil flow in the 3 GPM circut and so the pressure drops off. Since this is a 3 GPM priority circut, when the flow drops below 3 GPM there is NO flow through the oil cooler and that is the oil that provides lube for the transmission and differential. Continued use of a tractor with MCV pump problems often lead to rear end failures.
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