There are many reasons for brake failure. First, there is a lack of necessary maintenance of the brake system. There are too many impurities in the brake master cylinder and the sealing is not tight. The vacuum booster pump fails. The brake oil is too dirty or several types of brake oil are mixed and heated. Air lock occurs in the brake master cylinder. Or the sub-pump leaks oil, the gas storage tank or the pipeline interface leaks. The second reason is due to improper operation.

Guys, do you know what the size of the wrench is to unscrew the nut lock drum?

Sorry l took so long to get back to you what l can see that should be the fill plug for the rear.

Though it's not necessary, it's an excellent practice to remove the differential's oil before detaching the axle shaft and brake drum. By removing the oil, potential contamination and leaking are avoided. It could be achievable to take out the axle without draining, but doing so could be problematic and cause leakage from the remaining oil. Disposal additionally renders it possible to check for extra issues such as shavings of metal. For specific instructions, always consult the vehicle's service manual-certain types can have particular requirements. Having a tidy and organized approach a top priority provides more effective execution, minimizes the effects on the natural environment, and helps with avoiding costly repairs.

@Scott_, do you mean the cap shown in this photo (link below)?
https://files.fm/f/pt7xgxdgf3

Note this is not a car there is no rear cover to take off

Read the vehicle's repair manual for advice on how to take off an axle shaft and brake drum safely. It typically involves removing the differential cover, wheel, and brake mechanism. While not always crucial, letting the oil from the differential may reduce leaks and contamination throughout the entire process, permitting a more thorough more effective repair.

There should not be to much just put the rag in the end of the axle shaft this will stop most of the oil after you are done raise the mast up put a pole or something just for safety in the channel in the middle of the rear there is a plug remove it and put your finger in it and see if it is full if not fill with gear oil in the same hole.if it runs out of the hole it is full. Hope this helps

Yes thank you. You helped me a lot. If I may, I would like to ask about two more things. Will a lot of oil come out of the differential after removing the axle shaft? And the second question. Do you know how to check the oil level in the differential?

No remove the axle shaft and put a small pan under the shaft end to catch some gear oil after you remove the shaft you can put a rag in the shaft ehd so no oil comes out as you are replacing the brakes just remember to remove the rag before you put the shaft back. Also check the rear to return the oil that came out when you pulled the shaft. Good luck hope l was of help.