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The 'cooked' rubber hose had an almost invisible crack at rest. That was an insidious (?) elbow hose. One month prior, idle speed was set at 800 rpm and mixture screw was set using an exhaust analyzer at a local MOT facility, then a plastic seal was applied to it. Ignition system was checked with a spark voltage tester (an oscilloscope that shows all 4 HV signals at once) They made a great job, my engine runs smoothly, fuel consumption is 5.7 l/100 km at steady 100 km/h, 9 l/100 km in city on those very hot days. So that's why I believe the crack was minor when brakes were not applied.. Only when pumping the brake pedal, I think the crack behaved like a heart valve causing large vacuum leaks. @masster The hose was replaced for now with a strong heat resistant acetylene hose. I don't know where to find a similar elbow capable to resist to manifold heat for a long time.

I have 2 images for you after door panel is off. Hope it helps.

Problem solved with all of you help and some chance. Having the cause narrowed to 1-way valve (check valve?) I looked closer at that elbow rubber hose coming out of manifold (see photo below) and I saw a long veeeery thin crack on the firewall side (hardest to see). When The hose was pushed with my finger, the crack got bigger, splitting open. So I guess that when I pumped the pedal, the hose was flexing largely enough for atmospheric pressure to push in and destroy all vacuum thus stalling the engine. I've put a new rubber hose and now everything is ok. Of course, idle speed drops to almost 700 rpm under heavy pumping, but that's normal for a car without computer under such heavy braking load. Thank you all and sorry for my English, I am very excited that my problem was solved.

I see that I must go buy a new hose with the 1-way valve. Here I have a photo of power brake system from Britannica encyclopedia. Can you explain to me why the bad valve kills the engine, please? Because I took that hose out and if I blow from booster side, air goes freely, but if I blow from manifold side, air can't pass.

I've attached a vacuum gauge to PCV hose. Idle speed is steady at 800 rpm, vacuum needle is in the green zone (500 mm Hg or -670 mBar). The moment I pump the brake pedal quickly 5-6 times, vacuum is indeed totally destroyed. But HOW it is destroyed so severely so quickly, yet having normal braking when driving, beats me. I want to understand what is faulty and how things happen step by step before buying parts. What more tests should I do?

Hello to you, I am registered now and I need your help. My problem is this: with car stationary and engine idling, if I depress the brake pedal several times in a rapid succession, the engine will stall. Everything is fine under normal driving conditions, the brakes are strong, braking is easy exactly as it should. I could not find anywhere a reasonable explanation and cause of this problem. Can you help, please? Thanks.