how can I tell if my booster is really working, do I just disconnect the vacuum line to see a difference?

If you unplug the vacuum line to the booster and try to move the car under a little bit of gas and then press the brake pedal, you should find the car is much harder and slower to stop than normal. I would only recommend doing this in an area that you have plenty of room to come to a rolling stop, if needed. I would not recommend doing this in a driveway but in a large empty street or parking lot.

Hd Bob , while the motors running , clamp off the vacuum hose to the booster , no change in engine sound/ revs indicates no loss of vacuum which tells you the diaphragm in the booster is still sound … but … if I was you and your replacing the master cylinder , pull the booster and have it rebuilt ! A man can then sleep at night knowing his brakes won’t fail !!!

Considering how old your car is , change the 3 flexi hoses 2 in the front one on the diff and inspect all steel lines for rust .. and if in any doubt , replace them !

Edited July 24, 2025Jul 24 by 64 kiwi boni

I agree with Kiwi. I had my booster rebuilt and re-plated when I replaced the MC back in 2001.