1990 240 volvo,no brake lights and rain makes it hard to start

Both. If the running lights ( marker, tail, clearence, or parking) and brake lights are out since the rain, a tail light assembly is broken and filling with water enough to create a short to ground. If there's no broken lens, whitch normally only blows the bulb. Then, the problem lyes in the trailer light harness. the brown wire (tail) & green wire (left turn/brake or yellow wire (right turn/brake) must be bare exsposed that shorts to ground when wet. all the trailer wires could be smashed together damaged the plastic coating on them. no problem till water shorts to ground. The white wire is the ground wire make its not able to touch the other 3 wires or bare & wet. Tape wires seperate then replace tail/mkr fuse and brake fuse. That should do it.

could be low on fluid
could be worn pads

There is a switch located above the brake pedal, this switch can be adjusted. It can be hard to see, climb under the dash, and look up towards the steering column, check the switch for proper operation.

check for faulty brake light switch on steering column under dash

Yes. Check your body grounds also and make sure they are clean and tight.

yes. the light indicates a fault in the abs system (it will store a code which can be retrieved with a code reader), and the abs system will be disabled until repaired. your brakes will function normally, but you will have no abs feature.

So many lame responses...I went through this same experience and can tell you in my case it was the alternator. All of the warning lights such as the ABS light, airbag light, the warning about the brakes are all because the alternator is not providing the DC voltage all of the car's sophisticated electronics need to correctly monitor these subsystems' status. Mine left me high and dry by the side of Interstate 5 needing a tow. The low battery warning was off and on for about an hour before all of the other warning lights came on, and then about 5 minutes later every needle on the dash went to zero and the car died. After sitting waiting for the tow, it did start up again, but we got it towed to a dealer (thank god it got us within 8 miles of a dealer! the first warnings started when we were in the sticks) who diagnosed it as an alternator. $1200 and two days later and all is well again.

The lights indicate a problem with the transmission. The fact that the engine wont start, leads me to believe that the PNP (neutral safety switch) is the problem. You will have to have the vehicle scanned to make sure.

you can do that only will get fix if there is no actual problems with the hydraulic system sounds like a pressure diffrential valve problem but make sure sometime the radio might ask for factory code after the battery is disconnect.
good luck hope this help.

Turn on the headlights try to start if the lights go off or go very dull the battery most likely is flat

If the lights stay bright, make sure the transmission is in neutral and the park brake is on.
Hold the key in the start position and let someone tab the starter motor with a hammer, often the tarter engaged, The problem in this case point to worn brushes.

Good luck