Originally Posted by
TJBrass
The RAT does not pressurize the hydraulic system, it is a wind driven generator providing electrical power that, in turn, provides power to a electrical driven hydraulic pump. Most modern engines have some type of flow control valve that disconnects or reduces the hydraulics on a windmilling engine. The idea it that it is easier to windmill start without the extra strain of powering a hydraulic system
Again this depends on the aircraft type. There are hydraulic only rats on aircraft. The 757/767 is a example.