as long as there is some fluid in the tank, then your system should function. your fluid will run hotter but all should still function. just the tank being low or half full should not result in a sagging rear end. as long as there is some fluid for the pump to suck in and pressurize all should work...

same as low engine oil...if the pickup tube is still picking up oil, it's enough...but no room for error either.

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1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5