Oskar

Any comments?.

aztoyman

Need more info.

Sudden occurrence?
After working on brakes?
Could be a bad booster.

HSHSURF

Throw on the booster/master cylinder push rod could be out of alignment. But as stated above, need a lot more info to try and diagnose properly.

akwheeler

even though (as stated above) I don't know what you drive or anything else about your issue I'll relate some problems I have had in the past.
I bought my 95 4runner with a really hard brake pedal, no power brake assist. I replaced the vacuum booster and master cylinder as a matched set, problem fixed for about 10 years.
last month I started to have the same issue but intermittently, I believe it was because I did extensive smoke testing of the entire intake/exhaust and fuel tank trying to nail down a suspected vacuum leak that caused a lean code and the oil residue from the smoke was clogging the little passages (ports? orifices?) in the booster. punch line is, I replaced the booster (no longer available as a combo with the master cylinder). no more issue. Also, the check engine lite turned out to be from the new MAF I installed (3.4 swapped) not reporting correctly.
I hope this helps, check for vacuum leaks, see if the booster holds a vacuum, give us more info.