i recently replaced front and back brakes, after doing my backs the other week the pressure improved but feels like its slowly getting worse again. im going to replace the master cylinder but i saw there were some upgrades. what am i better of doing? thanks

 

I would first start by bleeding your brakes before spending money that you may not need to. When you do bleed them start with the brake that is the furthest from them master cylinder. Start with the rear passenger side then rear drivers side, front passenger, then the front drivers side. This is the correct order to bleed the brakes. I hope this help. Let me know if it works for you or not.
Good Luck

 

I already bled them after doing all 4