While cruising at 65 mph, and after about 15 minutes of driving, ABS and BAS/ESP warnings appeared. After shutting off engine and a couple of hours cooldown, drove car again and same thing happened after 15-20 minutes.
I looked under hood and noticed the hydraulic fluid resevoir located next to the windshield washer resevoir was below the second mark (apparently low). Could this low fluid condition cause the warning lights to illuminate? And can the fluid be obtained anywhere other than a Mercedes dealer?
My car is a 1998 E320 wagon, 2wd.

The low fluid level will not turn those lights on. That fluid reservoir is for the power steering and hydraulic leveling systems. As for as the fluid , use only the MB fluid to top it up. I would also suggest having the systems checked for leaks if the fluid level is low.
The most common cause of those warning lights is a failure of the brake light switch, but it could be something else also. You need to have the fault codes pulled to determine the cause of the warning lights.
Good luck, Robert.

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Mercedes Benz Master Tech - Retired

If you can duplicate that problem!!! Have the charging system checked at that point that the lights are on, a simple volt meter will suffice.

MAKE sure to use only the MB approved P/S fluid as that is different than another MB untill the 2000 & later cars. Pentosin CH-11 is the ONLY correct fluid!!!

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MERCEDES Benz Master Guild Technician (6 TIMES)
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Make sure you have the same size tires all the way around.