Go down and check coupons out. Remember to check price history if it is available. What is best price?
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
Aliexpress
There’re some similar (or not) items on Aliexpress
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. You can get coupon down below.
Link: Banggood product page
1. ($12.05): BGRepair63
coupon code has to be applied in shop’s cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
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
(these coupons are old, so probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for Brake Fluid Tester Pen.
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)).