Noticed the voltage shown on dash gauge was low after starting and warm-up (hand brake on as well as ‘brake’ light) - checked battery with voltmeter and measured only 11.7 volts. Ran engine 10 minutes and volts remained low. Charging warning light was not on - but figured it might be burnt out due to age. Went to NAPA picked up a new alternator. Was moving car to install and released hand brake (brake light goes off) and voltage on gauge went up - checked again at battery and it read 14 volts. Set hand brake again (brake light goes on) but reading remained at 14 volts. Turned engine off reset hand brake, turned over engine - again 11.7 volts. Released hand brake back up to 14 volts it goes again.
Took belt off alternator, set hand brake, started engine, no charge warning light came on, volt reading on gauge low. Tried to release hand brake - it felt like it was out of adjustment/loose and brake light would dim but not go off - all the while volt reading stayed low.
Put alternator belt back on and started engine - rig resumed working as it had been and hand brake (and light) again worked properly
Well, since no one has responded to this. I would recommend you visit the forums at ih8muddotcom–the land cruiser gurus there are quite knowledgeable.
I’m assuming that you have a fj60 (the 4-door wagon) since you mentioned a voltmeter. I have an 82 fj40 (older two door model), but this may apply. On mine, if I start it and just let it idle, the alternator will not begin to recharge the battery. It will only start to do so if I give it some throttle to rev above say about 1500rpm. After I do this once, it will feed current back to the battery. I know this because my ammeter will not indicate the battery is receiving current until I do so. Also, it takes a few minutes for the battery to recover from the drain caused by starting the engine.
The other thing that may apply here is that my brake light doubles as my charge warning light. The dim brake light you noticed when the belt was off is the electrical warning light on fj40s. It sounds like it is okay, but if you think there is a problem, you might want to check that the alternator belt it tight and make sure that the voltage regulator is working. Good luck.