I just replaced my Brake master cylinder on my 84 300DT, And i noticed my brakes are very hard now. I heard air rush past the seal between the booster and cylinder when i took the cylinder off. Is air supposed to get sucked in to the booster when the Brake master cylinder is removed or do i have a brake booster leak?

Thank you

That's a good thing.

You created a leak when you loosened the master cylinder. If the booster held vacuum until that time, it would seem that the vacuum integrity of the booster was good.

Simply replacing the master cylinder should not harm the booster. I believe there is an O-ring seal between the master cylinder and booster. Did you replace it?
I can't answer your specific question but I would hesitate to condemn the booster if it worked fine prior to the master cylinder replacement. What were the symptoms that made you replace the master cylinder?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Did you bench bleed the MC before you installed it?

No but, what would bench bleeding have to do with lack of boost? Would'nt my pedal be spongy if i had air in the lines?
The new master cylinder came with an o-ring. Ill take it apart again. Thanks

Not a thing.

If you had boost previously and now you don't, the most likely culprit is the seal between the m/c and booster. Or you knocked a vacuum line loose somewhere.

Also, make sure that the pin from the booster seated properly in the m/c.

You can check the booster with your mity vac. If you hook your mity vac to the main vac line and turn the car on. You should get between 20" and 25" of vac. If not then remove the vac line from the booster and plug it and read the mity vac. If it jumps up to the numbers above then your booster is leaking.

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