Inspect for a leak in the supply line from the rear end to the hydro and the filter seal. These are the points that are under negative pressure when the system is running. With rubber components, there may not be a visible fluid leak due to the rubber resealing when the negative pressure is not present, but they may still allow air into the system. Air can also infiltrate in certain areas of the charge pump housing seal.
Check the dipstick cap to make sure that water isn't getting into the rear end.
There are several fluid types used in these hydros. ATF type A (Dexron) and type F are both red and will change colour as mentioned previously. Most other fluids used are not red.