Why does my Caterpillar 140H have low speed in gears 1 to 3 but normal speed in gears 4 to 8?

[Gryks Machines]

I'm having a problem with my Caterpillar 140H. The machine shifts into all gears normally, but 1st, 2nd, and 3rd gears are very slow and weak. Gears 4th to 8th work fine and have normal speed. Could this be a low transmission pressure? Trying to figure out where to check first before replacing parts.