I've been noticing when I come to a stop lately that the brake pedal will noticeably drop down a bit, causing a very quick like stop. Feels like someone cracked open the bleeder real quick then closed it. I changed the fluid and had a helper assist me with bleeding the entire system. I noticed when i cracked the bleeder that very little fluid came out no matter how far i opened the bleeder. I checked all the lines for kinks, etc. but found nothing. No leaks at either wheel cylinder or caliper or where lines connect. The front caliper hoses cant be more than a year old. Back hoses maybe two or three years old. No codes or lights on dash.

Watched a youtube video on checking booster. I can confirm I have vaccum to the booster. Can also confirm booster holds vacuum by the sucking sound it makes when i pull the check valve out. Check valve also good. Brake pedal drops like it should with foot on pedal and starting car. Also, brake pedal with foot on it does not rise back up after shutting car off. With car off, after building pressure in the booster, I can depress brake pedal 3-4 with each press gradually becoming harder until no pressure assist anymore so i know there is still vacuum after shutting car off.

Last test, with car off and no vacuum in booster, I apply firm pressure to pedal and it holds.

Car running and I apply firm pressure and the pedal wants to sink. Doesn't go all the way down but if I push harder it will.

My first guess would be master cylinder. It's the original and I have 192,000 on the car. But why doesn't the pedal sink when the car is off? It's hard as a rock!

Can try bleeding again but I felt we did a sufficient job. Did not run the master cylinder dry either.