I have purchased a few months ago an s130 1982 2+2 turbo and i have a problem with the brake system. Sometimes while driving the brake pedal starts to get hard and the brakes start to engage, so i had to bleed the master cylinder on the bleeders it has, pump the pedal and it was good to go again. that issue also occurs when letting the car sit for a few days with no use, and i have to bleed the MC everytime that happens. I did a full flush of the fluid since the reservoir had alot of dirt, fully cleaned the reservoir and added fresh fluid, then bleeded from the front brake calipers (rear ones are not engaging or connected at all) until dirty fluid finished to come out, thinking that would solve the issue, apparently did nothing.. what could be the problem? the booster or the master cylinder? it brakes well when it doesn get stuck.. and also every time i bleed the cylinder a little dirty fluid comes out.

 

Bleed from the passenger rear caliper then driver then at the master. Repeat for the front.