I normally never have to use the wipers on the Cadillacs, as I never drive any car in the rain. But from time to time I do check if everything is o.k. with the wiper and washer system.
On the 74 Coupe deVille the “Washer Fluid" light came on when I turned on the wipers.
When I checked the fluid level, the bottle was full.
I then discovered that the float unit seemed to be missing some parts like the magnet, which activates a reed switch, and the float itself.
Luckily I found all the parts sunken at the bottom of the washer fluid bottle.

The unit from the 1974 Coupe deVille

When the fluid level in the washer fluid bottle drops below 1/3 full, an amber tell tale light comes on labeled "WASHER FLUID".
Engaging the wipers allows a small amount of current to flow from the wiper motor terminal through a yellow wire to the washer bottle float unit. When the fluid level in the washer bottle is low, the float drops, allowing the circular magnet to separate from the cap assembly. This permits the contact points within the cap to close, bypassing the resistor that is in parallel with the contact points, and causing current to flow to the indicator light.

I had two problems:

1. The float itself did no longer float in the washer fluid
2. The reed switch in the cap did not deactivate - it was stuck and the contact points in the switch did not open with the magnet.

I got some valuable tips on the CLC forum about what kind of switch is used inside the cap, and how to make the float floating again.

TJ wrote about the switch:

"I don't remember what year it was that I fixed but I don't remember it being that difficult.    There was some rivets or something that had to be drilled out or broken off.  I think I used screws and nuts to put it back together.   The working bit is what is called a reed switch.  They are by today's standards old technology but since there is a fair amount of room in there you don't need to find the exact same one. If you do a search for reed switch you should find em. It's usually a greenish glass capsule with 2 leads coming off the ends. "

After I applied an additional, much stronger magnet to the switch, it suddenly came back to life! For whatever reason it is working again. It looks like the stronger magnet opened the stuck contact.

I still had the problem with the sinking float though and got another great tip from Jim:

“After inspecting the assembly and pulling up on the float, the warning light no longer displayed, so I knew the float was bad.   I took the float off and dropped it into the reservoir.  It sank like lead, so I knew it was a porous material.  I knew I had to remove the moisture somehow, so I thought the safest solution was to slowly heat it up so the fluid would evaporate.  I figured it was designed to tolerate extremely cold and extremely hot outside temperatures, so figured the lowest oven setting should work.  I got lucky in that there was no distortion.  I wouldn't recommend leaving it in longer than a few minutes, though. " 

I tried the oven drying method mentioned in Jims posting above at the lowest setting. I left it in the oven like Jim recommended, and afterwards the float did not sink any longer, but it still could not hold the magnet up to the switch.
Then I put it into the oven again for an hour. Afterwards it got slightly better, but it was still to weak.
Once again I put it into the oven at a higher temperature (230 degrees Fahrenheit, or 110 degrees Celsius). Luckily the float does not warp or melt at this temperature. I switched the oven off after another hour, and  I left it in there to cool down over night.
It indeed is a porous material. It seems like the higher temperature in the oven "fixed" the float for me. It also feels completely different after the cure - when I touch it now it feels much more porous than before, when it felt like a piece of plastic.
It's floating perfectly again - I hope it will stay like this for a long time. If the problem reoccurs it´ll go back into the oven…

From the 1974 Factory Shop Manual

I checked the switch with a voltmeter.
You have to get the rivets out if you want to access the switch.
The reed switch is under this cap.

The magnet which is normally activating the reed switch.
When I tested the float it sank…

Tags:Washer Fluid Low Level Indicator, Washer Fluid Float, Reed Switch, Restoration, 1974, 1974 Cadillac Coupe DeVille, 1974 Cadillac Repair Tip