[Keller]

Do you have the grey or black switch atop the unit?

I would strongly suggest draining all the fluid from the reservoirs (maybe with a tube and bottle or with a syringe), then refilling with clean DOT 3, and then bleeding all four wheels (shortest to longest path) in order to get rid of air (and potentially condensation/water) in the system. That might improve brake feel considerably.

Of course, a lot depends on how worn the pads and discs/drums are. If they are still in good shape, doing this might help a bunch.

Observe that the fluid level has to be checked/filled with the system depressurized, as you have noted.