Brake Fluid Tester Pen (Banggood, Aliexpress: 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 (the best price with coupon)).
Price history, coupons and price alert are available on this page. Best price?
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. One coupon is available. Cool!
Link: Banggood product page
1. ($12.05): BGRepair63
just apply this code in your 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/feV
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
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Price history and price alert
Do you need price history chart (for all/most countries) for Brake Fluid Tester Pen? There we go. 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)).