Mercedes 308D seized brakes

[kitchenman]

Hi all, not Nissan I know but you may have the answer
My next door neighbour has an old 308D, bloke he bought if off said' now and again the brakes bind a bit, just rock it back and forth and it will be OK' He has had it a year
Well it ain't going nowhere and full power in 1st gear stalls the engine.
He had a look at the master cylinder, layer of simi clear gunge on top a sort of jelly, looks like clear silicon, fluid visible BLACK.
Brake cables both go slack with hand brake off, no signs of levers outside of back plates, so I do not know if that is the problem
My thought was master cylinder and was going to open a bleed valve on rear wheel, I got a bloke coming he said doubting my ability I think based on my advancing years. I have already overstayed my allotted time by 4.5 years.

Is this a known thing, as fluid looks past its prime I thought drain and replace and perhaps new seals, IF I can get the drums off. He borrowed my trolley jack and confirmed the problem is at the back, wheels off ground neither turn, rules out hand brake I guess unlikely both have jammed on
Any idea guys