Grain truck - 1979 GMC 7000 - fix 4 - way flashers????

I have replaced every wire and bulb fixture on the exterior of the truck chassis and grain box. That includes every marker, clearance, stop and turn signal light. Also replaced the light switch, horn relay and turn signal switch. Also put a new pigtail on the wires going to the fuse block for the flashers. Truck lighting wasn't maintained as it was used only at a grain elevator and not taken on the road. Odometer only shows 8000 miles. Everything works as it should except the 4 - way (emergency) flasher. Actually it did work last fall before I quit working on the truck before winter. Now it doesn't work. So, if someone really knows the 4 - way flasher circuit you can really help me out. OK - the left and right turn signals work individually. That means the turn signal switch has power for the the L and R circuits individually and that the wiring out is good. Does the 4 - way flasher have a separate power "wire" coming up the steering column? It does have a separate "blinker" on the fuse block. Therefore it probably does have a separate wire to feed the switch in 4 - way operation. I will check the fuse. I did switch the "blinker" with the regular turn signal "blinker" - it worked to blink the regular turn signals. I sometimes wonder if the fuse block is weak. There was a time when I had the block unscrewed from under the dash and had to wiggle it to get the tail lights to work. I wonder if it isn't getting power from the fuse block / "blinker"? Since the R and L signals work and the turn signal switch is new (and the 4 - ways did work earlier) it is probably is a power issue. Any ideas or diagnosis would be appreciated. Thank you.

Yes the 4 way has a seperate power supply, that is why you can use the 4 ways with the key turned off but not the turn signals.
Pull the flasher and use a test light to see if the socket for the flasher has power if not your problem is from there back.
If it does have power then your problem between the flasher and or the turn signal/ 4 way switch .
Dave

Another thought
Some of the aftermarket flashers, the HD clear plastic ones were too big to make proper contact when installed in the fuse box, but the factory style little tin ones would work fine.
Dave

As Determined said, there is a separate power supply for the four ways. It starts with a 15 amp fuse connected to the battery buss bar in the fuse panel, then to the hazard flasher.From the flasher power goes thru the brown wire in the ribbon cable up to the turn signal switch. Turn signal power goes from the ACC buss bar thru the t/s flasher and the purple wire in the ribbon cable. Just an FYI, although I don't think it applies here, there is a third power feed to the T/S switch that comes from the stop light switch on a white wire.

Check the fuse clips in the fuse panel carefully. They are notorious for rusting and loosing spring tension if they have ever been overloaded. The wiring on these trucks was barely adequate for the factory installed lights. If the truck has ever had trailer lights connected to it, all bets are off, anything could be open, corroded, or melted.

You know how yours worked before and I don't,but my Ford CT 4ways only work when you turn the switch(Hazard)on and then move the turn switch either L or R making the circuit live.Dumb,but worth a try.

Thanks for the information. Yes, the fuse block doesn't really look the best. Years of bees wings from being around the elevator. The bees wings probably collected moisture. Is the fuse block repairable - pulling the metal connectors/studs out and replacing them like on a corn planter montior harness?? I will check for power at the "blinker" on the fuse box. Thanks.