300 tdi brake issue

Hi ive had an issue with the O/S/F BRAKE CALIPER brake pads where low so changed them but had isse with brake pedal fading away to nothing at times so i took to bits and noticed piston not working so renewed pistons and seals but seals where a nightmare and but after change still had a brake fade so i thought was brake disc as was very thin so changed that too it worked but thewn started to fade again so ive just checked and one of the seals wasnt in correctly so instead of all the hassle ive changed the caliper,and seemed to work but today it faded again so took it off again and checked pistons and they are working fine moving freely so i checked brake fluid and its only coming out as drops not a squit which i would expect does anyone know why or how i sort this im thinking master cylinder but then agin it works ok for a week or so or untill ive travelled for a while it starts toi fade away also what is the unit between air filter and water bottle that the brake pipes go into could it be this???

thanks

Karl

also ive just been out in it and its braking on the other 3 wheels but still not working on the OSF

daft question but is the bleed nipple on the caliper at the top of the caliper or the bottom. Also have you checked your brake flexi hoses

bleed nipple on top and flexi fine

if you can park it for 12 hours .......... park it with the front end 2ft higher than the back, loosen the master cylinder cover after 12 hrs all the air bubbles will be out of the system

i dont think it air im thinking towards master cylinder rubbers gone and fluid pushing past so not working correctly but thank you good tip ill try before i start on master

Maybe vacuum pump? Also, are the wheel bearings ok, as wobbly bearings can cause this.