Best bet is to make sure you spec the pad for your particular caliper.
The manufacturer is stamped on an open space but you have to clean it up to see. Sometimes not that obvious.

Might as well replace the pads. Then you can eyeball the calipers for leaks.
Pad wear patterns can also shed light on calipers behavior.

I forget what's the idea behind the rear reservoir..... I only recall that one must be sure to top out both after refilling.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD