All four brakes sticking on

Got a problem with the brakes on my '91 850i.

Brakes were normal when I started off this morning but as the day drew on (100 miles plus) they started to bind on untill, on the motorway at around 70mph (Officer) they started making a loud drumming noise and the car shook violently when the brake pedal was pressed.
I pulled over for a few minutes and all was well although they now stick on all the time. No need for a handbrake now!!!!
The discs have all blued so its all four calipers on at once and the pedal is rock solid with no free play as they normally have although the brakes work ok at normal speeds.
Does this sound like master cylinder? Possibly the Bomb?
Or could it be a simple brake fluid flush or pentosin?

What would make all four brakes work uncommanded at the same time?????

Anyone had this before???

Thanks for that TerryY, Ive got a spare booster so I'll try that first.

Havent got any Pentosin fluid though!!!!!

Any chance it could be the master cyl????????

Checked pedal travel and its not that so must be a component failed.

I think its time to get the tool box out.

Sometimes you just land on your feet.

Or I just did!

Needed to sort out this brake sticking issue and so ask BMW Stealer what the cost of a master cyl would be..... £412+17.5% tax!!
So I used a part finder website for a m/c as Ive got a spare regulator,bomb,booster etc and found one used for a very reasonable £70 delivered. It showed up today with the brake booster still attatched!!!!
Did the O ring fix on my old one, my spare and the one delivered with the m/c, not difficult and cost a grand total of £1 for all three.

Took everything out of the car yesterday and have got booster and m/c back in the car but still need to plumb in the bomb/regulator etc but not a difficult job at all.

Also got 2 liters of Pentosin 7.1 from the Stealer for £8 per can which was the cheapest quote for it that I got!!!
BMW? Cheapest? Cant be right!!!!!