Jim,
If you suspect the master cylinder / booster is sticking, try repeating the condition and then cracking the brake lines loose at the master cylinder. If the brakes 'unlock' then it's not the brake hoses.
I had the vacuum booster fail on my 1972 220.
It would hold pressure to the brakes with increasing force as the car warmed up, to the point where the car would hardly move! In my case, this was accompanied by gradual loss of brake boost as the car warmed up. A salvage yard booster finally fixed the problem.
Hope this helps.
Happy motoring, Mark

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DrDKW