Brake booster problems

About 5 months ago, while driving Brake, ABS, Traction, slip lights suddenly turned on in my Prius 2010 - 165K miles. Braking was also weak. I could hear the whirring sound from the brake pump every few minutes. Error code was C1391, the mechanic replaced both the ABS Brake booster pump (accumulator) - part#47070 47060 and Brake master cylinder (actuator), part#47050-47140 and calibrated. To save money I had them replaced these with used parts I purchased from eBay. Everything seems to be working for three months and ~1000 miles. But the same lights came back again. But this time I do not hear the whirring sound from the brake pump. Brake pedals feel hard but I can brake and stop the car. It brakes better than last time I had the issue, but do not feel it’s 100% right. Same mechanic repeated the recalibration. But it did not fix the problem. He thinks the replaced parts might have failed. Since I procured the parts he wouldn’t warranty the fix. But given that I am not hearing the whirring sound from brake pump like last time, I am doubting his diagnosis. I already spent over $1200 last time and I am back to where I was with this problem.
Another strange thing is these warning lights occasionally goes away for a few min but are on most of the time and comes back on after I restart the car. There should be no issue with the 12V battery as it was replaced 5 months ago (around the same time I had the braking issue originally).
I really need to have the car working for couple of more years. Any suggestions on what I can do or check to figure out the problem.

 

OP here. Thanks for all responses. In my case, the parts that were used are correct and are verified by the mechanic. Mechanic is also one that specializes in Prius (Hybrids). Also it is not that the changed part is bad to begin with as it worked for some period of time. It could be my bad luck that these parts have gone bad twice. What is annoying is that we have to change both parts together every time. There seems to be no way to diagnose which part is bad - as per this mechanic.