Can a Brake Booster go suddenly bad? No PRM rise

[Miloslav Maun]

I have this used booster from ebay. In the beginning it worked fine but suddenly it kind of stopped working. It is now very hard to press the pedal and especially when a higher braking force is needed the pedal must be pressed very hard. I had bad boosters before and when I pressed the pedal, the idle RPM always went up by 500 or more. Not in this case. Can it go bad like this? I've checked the vacuum hoses, it seems that there is enough vacuum at the end of the hose (and the air leak when the hose is disconnected is massive)
BTW: The master brake cylinder is new and all other parts are perfect. Car: 1966 230 SL, T51-200 booster.
Does the newer type booster T52 work in 230 SL?

Thanks.

[Benz Dr.]

A brake booster can go bad without any real warning although there's usually some signs that they're not working properly. I find that the pedal may be hard at start up but then it returns to normal shortly after driving away.

 From what I remember, no, you can't use a 280SL booster on a 230SL. The lenght of the push pin inside the booster is different and the brakes will lock up after being used a few times. 

[Miloslav Maun]

Thank you, Benz Dr.

Unfortunately even after 500 km of driving the booster did not start working.
So I installed both of the bad ones which I think I remember how they behaved. They had the classic symptom of idle RPM rising when the pedal was depressed. Strange, the old ones did not do anything either. No boosting, no rpm rise. Like if the vacuum wasnt there. But it is, if I remove the hose from the booster, the engine almost stops and there is quite some suction on the hose.
Can there be some obstruction of air leak somewhere? How strong should be the suction on the end of the hose?
I can head a hissing noise at the engine. I cannot locate it exactly but it seems to come from the front of the engine near the air filter or idle bypass screw.
Any ideas?
Thanks