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By chance are you doing a lot of riding on dirt or broken-up damaged road? I noticed your Indian flag and I understand the roads aren't the best there. It could be you're just sloshing up fluid on bumpy roads and pumping bubbles into the system every time you brake. If that's the case you might remedy the problem by filling up your brake reservoir completely with DOT4 (or appropriate type) brake fluid as close to full as possible without spilling. You must remember to remove the fluid before your next brake change. You will also be able to determine if you're leaking any fluid out of the system. At least you can rule out fluid-loss or air penetrating into the system.

Maybe you bumped a banjo-bolt loose or unseated a crush washer. You can buy them in bulk cheap. Maybe confirm the fluid is the correct type and not eating the seals on the pistons on the levers and at the calipers.

If you're not losing fluid but still getting air, my money would be on bumpy roads and pumping bubbles into the system.
 

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Strange. If air is getting into the system through the master cylinder, fluid should be leaking out through the master cylinder. The fix is probably just a simple rebuild. Maybe air isn't the problem after all.

If you have the know-how to pull out the piston, you could inspect the two rubber rings. It may be possible to rejuvenate them.
 

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...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.
I'm pretty sure the problem is your mechanic.


Remaining issues (assuming the mechanic did his job correctly): Not using crush-washers on the banjo bolts, reusing old crush-washers on the banjo bolts, not torqued correctly at the banjo bolts, not recognized a worn out hose, not recognized a cracked caliper, not recognized leaking piston seals, not recognized pitted pistons.

I would recommend replacing the caliper seals, it's an easy job if you know how to bleed the system. It will give you an opportunity to physically inspect the caliper, rubber rings, and pistons for pitting. I'm surprised he jumped right into replacing the expensive stuff rather than just give it the required maintenance.

This is why force myself to learn how to do this stuff. You are entitled to a refund, or at least have him return the parts he replaced. If he is like most mechanics this won't happen and he is happily spending your money as I type this. You paid for a qualified mechanic to diagnose and repair the problem, but you received a guy that would only make guesses. You intended to pay for a repair, not an attempt. I'm sorry you're having a bad experience. I wish there was more we could do for you. Without actually pressing the handle to see what you are experiencing, what you describe sounds like air is slowly getting into the system rather than fluid pushing past the rubber seals.
 

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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.



Map Font Parallel Auto part Illustration


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.
Map Organism Font Slope Parallel



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.
 

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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 ?
Seems like you solved your problem. I do not have experience with ABS devices but you can find an entire ABS Pump Unit Module for less than $100 (USD) and just the pump for about $75 (USD) on ebay (also USA). You probably have enough experience to ride without the ABS unit running.

If you have Electronic Contact Cleaner, you might give the contacts a spray down and see if it's just the contacts needing a freshening up.
 

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You are not accurate.
He took the fuse out, and he claims it stopped the leaver from sinking.
So maybe the problem is: DELAY VALVE 46300-MFG-D21 (NEW for ~107$)???
So there is no point in rushing to announce that either.
LINK to PARTZILLA
View attachment 46069
If the suspicion falls on the ABS system, a systematic diagnosis should be made for that system.
Of course you will also start replacing parts. Both methods are good.
What a complicated system. This video describes a condition that causes the lever to sink to the handle and not work (starting at 3:50). There are several pumps, motors, and valves in these systems. Understanding how they work will help in diagnosing the problem.

 
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