Just out of interest .....if you look closely at the cap on the brake fluid reservoir there should be a small plastic "pip" in the centre {there is on my 1988 2CV anyway } ....
By pressing down on the said "pip", it should light up your "low level" light on your dashboard.....its possible to reach the "pip" with ones finger whilst peering through the window at the light .....{on a RHD 2CV .....I dunno if its possible with a LHD car}
best regards
T.

In the past I have just removed the cap from the reservoir in the past then checked the dash light to see if it's working.

My biggest complaint is crappy brake light switches. My Trabant uses the same brake light switch (10mm thread) as VWs and other cars that screws into the end of the master cylinder.

I've replaced about three of these on my Trabant 601. The brake light switch either stops working or the brake lights mysteriously start burning while the car is sitting in the garage. I ended up moving the wire from the "always hot" fuse to the "hot when switched" fuse in the fuse box just for this reason.

Most replacement switches are made in 3rd world countries and last about 2 or 3 years. I've heard some VW owners complain the switches have actually exploded if you hit the brakes too hard. The last one I got was $8.00 and has a lifetime warranty! That's no consolation if someone rear ends you due to a faulty brake light switch.

I thought about using a cruise control deactivation switch from a Ford which is also 10mm but those switches had issues where they started leaking or the switch would catch fire.

The one on my 96 Ford Explorer started leaking one day so I picked one up from an autoparts store. I was going to replace it on my off day but before I got a chance I got a recall notice from Ford that these were defective and to bring it by the dealership and they would replace it free of charge. Imagine that!

Try a dud brake light switch on a 2007 Cit. C4 Coupe .....I've replaced mine three times in ten years, the brake light switch goes kapput and then the ABS & Traction Control lights come on too as a result.
Takes ya an hour to find the switch [the first time] too ......'cos its in the passenger footwell.
Theres progress for ya .....
T.

And yet I've never had a dud brake light switch in a 2cv in 25 years. Go figure!

Not even in 33 years and those of the two 57s are original also but they are of an other construction. Rather flimsy i will say...

Having had a brake fluid leak I can confirm that if your fluid reservoir runs low, the light will come on.
Don't all modern cars have a built in 'test function' - when you switch them on all the warning lights illuminate briefly.
On a 2cv it's s duel circuit to test that the circuit is working which isn't a bad idea. If my brake fluid runs out, Im glad I'll know about it

In my single experience the light comes on the very instant your foot goes to the floor. The " you should stop now, but can't light"!
In my case it was because a rough weld on one of the rear arms had worn through the rear brake line.

I don't know about cars sold outside the US but on every car I've seen as far back as to the early 1970s, there is usually a time delay relay and a either a buzzer or chimer that goes off for about 10 seconds that also turns the warning lights on after starting the car. My 1980 Fiat Spider does this. Non US spec Fiats of that same era did not have a buzzer to tell you the door was open. Must have been a US DOT thing. On my Fiat, the buzzer was activated by contacts in the ignition switch grounded through the door jam switches.

I disconnected the wires to the chimer and used the contacts (two white wires) from the ignition switch to turn the radio on as soon as the key is inserted into the ignition switch since there is no accessory position to just turn the radio on without the engine running.

On most modern cars, all the lights light up while cranking the engine over then after a couple of seconds the lights go off.

But no I've never seen a test light button for the brake fluid reservoir level on any other car than my French spec 2CV.

Most likely because the brake light switch is not mounted in the end of the master cylinder where it makes contact with brake fluid but on the brake pedal.

The brake light switch on my Fiat and Yugo are mounted on the brake pedal and I've never had to replace a brake light switch on any of those cars.

I was once working on a 1992 Nissan and couldn't get the transmission out of park. Turns out there is a switch on the shifter that has to receive power from the brake light switch in order for it to release. This is to prevent from pulling the car out of park unless your foot is on the brake pedal.

I removed the cap off the reservoir today to see if the low brake fluid light would illuminate. It doesn't burn. Just like the test light. I changed bulbs but it maybe an earth issue. I probably knocked one of the wires loose trying to remove the instrument cluster when I installed that LED bulb for the speedometer and gauges.

I'll check it out when I relocate the main beam, turn signal and hazard light above the test light switch.