904 Reverse/1st gear issues

Do a pressure test. That servo should have 230 to 260 psi. It sounds to me like a very loose band adjustment, or a low fluid-level,or a circuit issue. If you don't have the pressure with a proper fluid level, your pump-circuit has problems, and you will soon have issues in other gears as well.
At the absolute minimum, it could be a maladjusted manual valve; check the sync to the shifter.
In reverse all regulation is bypassed, including the governor; the gauge is reading maximum pump pressure.