Two things to check.

Check your brake fluid level to begin with.

Also there is a rubber boot on the inside of the brake reservior cap. Sometimes that cap will turn inside out and hang down in the reservior and push on the float switch.
Check the cap and push the boot back up inside the cap, if that is the case.

1995 Trooper LS auto 3.2 DOHC /w SOHC intake
1989 Trooper 2.6 auto
1989 I-Mark RS DOHC 1.6
1991 Stylus XS DOHC 1.8