Sounds like you have found your problem with the low fluid, now all you have to do is find out why its low.

Also, I think I would check the power steering tension. At the low speeds the pump is supplying the power steering, but may not fully be supplying the brake boost. The belt could be slipping.

You could also have glazed brake pads, there may be plenty of lining left, but if the surface is hardened, the brakes will suck, low speeds will be the first place it shows up. Gene