Yep, you are pulling a vacuum in the return line, and probably in the hydraulic reservoir as well. Here are the common causes:
First check your trans/hydraulic fluid to make sure it is not contaminated with water.
If it is, the oil on the dipstick will look like chocolate milk instead of clear oil. Contaminated oil gets stringy and clogs filters.
Next check the vent on the trans/hydraulic reservoir. Your hydraulic reservoir is vented - they all are - and collapsing the return line will also happen with if that vent is clogged.
But if those quick checks show OK then the most common cause is that suction filter between the hydraulic tank and hydraulic pressure pump is clogged & not feeding as much as the pump would like to draw. Your power steering is probably jerky and the 3pt or loader slow as well.
Not sure about your tractor, but most makes have the same two filters: a cleanable screen in the hydraulic outlet from the reservoir and also a spin-on type filter in the line to the pump. Clean the one and replace the other.
One of those things should do it. Might as well do them all.
good luck,
rScotty