Hi Hauser,
I think the discoloration is permanent and thus not reversible. In fact, from what I've read, color changes in many plastics such as HDPE, LDPE, and PP (the reservoir is most likely one of those) can be a result of heat oxidation, and or oxidation of the chemical structure, or breakdown of the stabilizers in the plastic. Soaking in degreaser might actually be adding to the problem rather than curing it.
Plastics have a variety of additives to them--plasticizers, anti-oxidents, UV and other inhibitors, etc. These outgas over time and also are affected by sun, chemicals, heat and other environmental factors.
The darn things just yellow with age and think there's no turning back.
I have various kinds of plastic tubing used for chemical feeds at my car wash, and they get rock hard, and change from clear to yellow to dark brown in a matter of months. I once found a substitute that didn't do that but it collapes in suction use, thus was useless. It was also gawdawful expensive.
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America