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I have a cbr 250r Abs 2012.
So apparently I was riding 2 weeks back and my front brake loses the pressure like half the lever was sinking inwards. I approached my mechanic and he suggested me the same to change the master cylinder repair kit I found it and got it changed. After changing it in about 4 days and lever sinked again. I got another master cylinder repair kit and got it replaced thinking it be a fault repair kit. Same happened again and this time the lever lost all the pressure, pressing it fully had zero impact on braking. Then my mechanic suggested to change the whole front brake assembly (reservoir, switch, master cylinder) i got the whole kit and got it replaced and after 6 days my lever still loses all the pressure. Have zero clue what to do now. Seeking answers i found this thread. I hope some of you might be able to help me. Thank you brothers.
 

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Danny, your mechanic is right.
It is possible that the new kit will be damaged.
It is possible that the second new kit will also be damaged.
It is possible that you will replace the entire unit for new one, and it will still be damaged.
Is it reasonable? The answer is: No.
Maybe it should be recorded in the Guinness book?

If there is a loss of pressure and the lever sinks, this only happens in one of the following situations:
(1). There is a leak, and brake fluid is coming out of the system (dripping out).
(2). There is a compression leak in the pump and fluid returns back to the reserve tank.

In your case I understand that there is no leakage. So what do you do now?
Answer: What you did before, sorry, your mechanic is right.
At some point, statistically, you will be able to buy a good new kit.
I will try a trusted kit tomorrow, and will keep you guys posted. Thank you so much for replying
 

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Highlighted in yellow are the remaining parts which can leak. At the caliper in photo 1 is the most likely problem. Item number 1 (piston seals). These are supposed to get replaced periodically. Item 5 often fails as they get pitted over time. Item F-5 (Caliper) can get chunks of dirt in them especially when you push in dirty pistons when changing brake pads. Item 3 (bleeder bolt) can introduce a leak. Failures of any of these part can let fluid out of the system or let air into the system.



View attachment 46067

In photo 2: Crush washers (part 18) are disposable and should only be used once. Trying to re-use them can prevent them from sealing. Banjo Bolt (part 17) can have damage allowing air/fluid in or out. The hose on part 2 can be aged or damaged releasing fluid or allowing air to enter.
View attachment 46066


Brakes look complicated but they are a very simple device when you understand how they work. 99% of the time they just need a rebuild. Since the master was rebuilt already you just need to do the Caliper. This just entails replacing the 4 rubber rings, inspect and wipe the pistons with a clean towel, and installing new crush washers. Pistons (part 5 photo 1) should look shiny and polished like your forks. It is cheap maintenance.

EDIT
It is possible the wrong DOT brake fluid is eating away at the rubber seals.
Thank you so brother for your input, i really do appreciate your effort. Can't be more happy to receive such a support.
.
Update :- removed the abs fuse. (abs not working now) ran the bike around 30kms the brake isn't losing pressure anymore.
What can be the probable issue in abs?
Can abs be repaired?
Or do i have to look for new abs unit ?
 

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Cahuna & BG covered the issue very nicely.
If I understood, the thread opener does not have a problem with brake fluid leakage.
I'm not so sure about "easy job".
According to the manufacturer's instructions:
Handling the brake system requires a mechanical skill.
The brake system is a safety system and there you have to be even more careful.

If I understood correctly, the thread opener buys parts himself and does the work himself, and has a mechanic that he consults with. But maybe I misunderstood?

As long as it is correct DOT brake fluid, it will not cause any damage to the rubbers.
The manufacturer sets the standard for our CBR250R(2011-2013) brake fluid to: DOT3 or DOT4
It is forbidden to mix different types, and must be replaced every two years.

Brake fluid absorb water to it, so failure to replace it on time can destroy(by rust) the metal and aluminum parts,
and destroy seals that wear out because they friction on the damaged metal surfaces.
Hello sir
Yes i buy the parts myself but installation done by a professional. I do consult him and buys the part he ask me to.
A pay close attention to everything he tells me I question him and look out for what he's doing.
I only use dot 4 brake fluid from a reputed brand and from a sealed container.
Even before my bike had this problem i regularly changed brake fluid every 9th month as i mostly ride in city traffic.
 

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Hi
"i buy the parts myself but installation done by a professional"
I don't usually criticize other mechanics, it's unfair and unethical.
Just be aware that your combination that you are telling us about does not give you effective results,
and even puts you at great risk.
You need to think on other combination method.

No one has any intention of saving money and putting themselves in danger because they saved money.

You cannot continue to ride your motorcycle when the brake system is not working:eek:
You might not know this but in India here there's problem of recieving replica of the spare part you are buying. That's my i go the extra mile to make sure that the item i am gonna get installed in my vehicle is original and genuine product. Hence i buy the spare parts myself. I am sorry if i mis- understood what you were trying to say.
I really hope that you would help me resolve the issue.
 

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I don't think your situation is different from anywhere else in India, or anywhere else in the world.
No mechanic likes when you buy your parts yourself and bring them to the mechanic to put them on your motorcycle. Because mechanics have an extra profit on the parts.
You want a cheap repair, OK, get a cheap service. I think everything is fine(y)
That's why I suggested you to keep doing what you're doing so well. all is fine.
Oh no bro the mechanic i get my vehicle repaired isn't cheap comparatively or put it this way that he's probably one the most expensive mechanic in the area. Also he ask the bike owners to get the parts themselves because if any parts fails in near future it's not mechanic's responsibility.
 
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