FORD 1998 Explorer - Manual (Page 165)

Do not let the reservoir for the master cylinder run dry. This
may cause the brakes to fail.
CLUTCH FLUID (IF EQUIPPED)
During normal operation, the fluid level in the clutch reservoir will slowly
rise. If the fluid level drops, maintain the fluid level at the step in the
reservoir.
Use only a DOT 3 brake fluid designed to meet Ford specifications. Refer
to Capacities and specifications.
1. Clean the reservoir cap before
removal to prevent dirt and water
from entering the reservoir.
2. Remove cap.
3. Add fluid until the level reaches
the FULL line.
Maintenance and care
165