Brakes: Brake booster failed. Hard pedal. ATE unit with DSTC

[Alberto]

This thread could apply to any Volvo diesel model, but in particular S80, S60, V70, etc. sharing the same ATE brake booster. Car has DSTC/BAS system. It's 380.000 Km. now.

Symptoms
Suddenly brake pedal was hard to press, and required extra effort to stop the car.
Possible causes
Vacuum pump, brake booster,
Steps
Check vacuum on the pump, rapid increase, correct
Check non-return valve, failed, no action, working like a pipe.
Suspect brake booster.
Get a new Booster and replaced existing with the help of some tutorials and a long 13mm. socket. Not very difficult, just time consuming, half a day.
Replaced non-return valve with a new one.

Result
Car now brakes as new, so problem was in the booster.

Overhaul
To find out what was the offending part of the booster, what I did was to open the bad booster and try to find the problem and the cause.
The pictures show the many bits and pieces the unit has inside. Most of the parts were in good condition, but one diaphragm was broken about 2cm.
Further investigation lead to find oil residues around the bottom part of the booster case, in contact with the broken diaphragm.

Reason for the fault (guess)
My guess is that as a consequence of the broken non-return valve, oil from the vacuum pump /engine was entering the plenum chamber, affecting the EPDM parts, in this case the diaphragm, and causing the hole.

Handy hint /recommendation
Test and if in doubt change regularly the non-return valve, that is simpler to replace and much cheaper than a brake booster.