964 booster or abs pump gone?

[tfwvisme]

My poor 93 964 C4 has been in the garage for over a year as I thought the brake booster pump was shot ( its not a daily driver ). Today I finally removed the the booster pump. and bench tested it, the motor in the booster turns over OK. So now I'm thinking it may be the ABS booster.
How it failed, 1st off, when I start the car I usually get a a buzzing/vibrating sound while the brake system pressurizes ( This is normal I beleive ). One day I went to start it and the buzzing never stopped, so I shut the car off ) and took another vehicle. I later went to check on it and the pump no longer made the noise and my brake pedal is hard as a rock. I presumed it was the big pressure booster, but now after bench checking the operation of the booster it may well be the abs pump?

Which one of the two should make the buzzing noise while pumping, as the booster pump turns but is very silent..
I need to figure out what part to replace in order to get her back on the road again.

[P3 Automotive]

Your Car should have only one pump for boosting purposes which is in front of the spare tire. The pump you see on the abs module is only activated when the abs is kicked in during wheel slip. If the pump in front of the spare tire is running too long you should normally have a warning light on your dash until the pump stops running. i believe the warning light is the one that shows the all-wheel- drive system. There is also a warning beeper when the pressure on the hydraulic system is too low.

If the pump doesn't stop running at all, i'd suspect either a good sized pressure leak or a bad pressure switch. The pressure switch is on the pump and has a rather large square plug with 5 pins. the pressure switch is usually anodized green.

the other possibility is a bad pressure accumulator if the pump seems to run for a long time every time you start the car but eventually stops running. If you pump the brake pedal a couple of times and you get the warning beep, this is also a sign of a bad accumulator. The accumulator should hold enough pressure to allow you to push the brake pedal 20-25 times before it is discharged with the ignition switch off. You can feel when the accumulator is discharged because the brake pedal becomes really hard.

[tfwvisme]

Well I probably have a leak somewhere as I would have to add a small amount of fluid periodically, and I beleive something in the hydraulics let loose when this last thing happened.

But the question remains, even though the pump turns when power is applied on the bench ( and in the car presumably ) why is it no longer making the buzzing pump sound ( the noise like a small air pump it used to make is gone) could it just be because the system is no longer under pressure due to the presumed leak.

Does everyone else's hydraulic pump make a buzzing sound when operating ( when starting the car and initially pressurizing the system ). Myne was like that for the 10 years I've owned it until the whole system stopped pressurizing then it went silent, which is why I thought the pump was gone.

thanks for replying