528e 1986 MC Bench bleed

[Notelect]

Awhile ago my brakes started losing pressure, I bleed the brakes using the two person method and got pressure back but while test driving felt it slowly loose pressure again. Later the pedal felt more screwy, like something was giving way inside the MC, then the brakes got worse and then I tried to bleed them again but nothing would come out of any of the brake rotors, in spite of the reservoir being full. Then the brakes went out fully, no pressure. Also I could get some fluid out when using a pressure vacuum pump, but pumping the breaks didn't result in any fluid leaving the rotors.

I checked the brake lines and there are no leaks under the car.

Kinda realized now I am winging it but I believe its the mc so I removed it and took one from a parts car, I heard I will need to bench bleed it but have found no guides about doing so with the e28. Is there any guides or is just bleeding the brakes doing normally enough to bleed the MC?

I am worried about trapping air in the ABS, if its not the MC or brake booster however I am guessing the the issue is with the ABS.

[4DSC]

Sure sounds like master cylinder failure from here. I don't think bench bleeding is required but will make it easier to bleed the system and get trapped air out of the MC once it's installed. Nothing special about doing it on an e28. Put the reservoir onto the cylinder, take some old brake lines, thread them into the outlets, and loop/hose them into the reservoir. Fill it up and just pump it by hand until what's coming back out is air bubble free. Then install. I wouldn't worry about the ABS too much. I didn't have any issues bleeding after I rebuilt my ABS unit. If you do have issues with air in the ABS IIRC you can trigger the unit manually (check ETM) for each wheels circuit to jiggle any air bubbles free while bleeding (at least that was my theory, been a few years since I did this).

[Mike W.]

If you have nothing coming out of the calipers it strongly, very strongly suggests bad hoses. Brake hoses have been known to swell internally to the point where no fluid will pass thru. I had that happen to an E3 long ago, it got to the point where I couldn't get a drop out, even with standing on the brake pedal. New hoses, actually used hoses off a parts car just two years newer and all was well. They're not even particularly expensive, especially compared to a new M/C, I'm seeing them for under a ten spot at Rockauto, or under 30 each for ATE from Autohaus.

Crack the line going to the hose on the body side and see if you can get any fluid out. Now it's possible you also have a bad M/C, but if no fluid comes out I'd bet it's the hoses.

[gadget73]

The big clue its bad hoses is the pedal will be firm, but it won't move fluid.  If the pedal just mushes to the floor its not making any pressure. 
I have definitely had bad hoses, but having multiples fail together in a short period of time seems unlikely, though not impossible. 

Secondary confirmation, crack the line loose right at the MC and see if it pumps out of there when pushing on the pedal.