Precise 5 gear test, It is powered by 1 AAA battery (not included), green light means normal, yellow light means the oil still can be used, but you have to pay attention to it, red light means you have to change the brake oil.
Just check coupons out (with price alert and history). Do you want to know what the best price is?
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
1. ($12.05): BGRepair63
paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): BgAuMotor, TTxBG10, AM15prime, AMPrime01, Automo, Automo1, BGCHRISTMAS, BGCARPA, BG20CAR, BGAutos, bGhOnEy10, BG2dfcd7, BGCARELEC, BG32673, 13f142, BGAFF10OFF, BGCuponation10, BGHONEY2110, BG10OFFAFF, BGwhielabel, BGHRepair2, BG10OFFAFF1
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Here is price history (for all/most countries) for Brake Fluid Tester Pen. The lowest price was: $2.99.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).