You could try 2 cups white vinegar in 5 gallons water. Wash first using a generous quantity of Dawn original dish soap. Let the suds soak in for about 5 minutes, then rinse well. After the water runs clear from the soap, SPONGE the vinegar water into the mane and tail. Use it ALL.
I did DanD (he's silver bay with mostly white mane/tail) in March and he's still sparkly white except for the tips of mane and tail. Keeps tangles down too.