2015 Silverado. There's no leaks and not losing brake fluid. When I back up and brake, it makes a funny sound and the pedal is soft. Going forward, it stops just fine if I haven't been on the pedal for awhile but if in stop-and-go traffic and riding the brake it will eventually go to the floor and be squishy. Then I have to pump the pedal to get brakes and they still aren't good. Drive for awhile and hit the brakes and they stop fine. Could this be anything else but master cylinder? Again, theres no leaks.

Most likely an internally leaking master cylinder.

Or brake booster, I would double check and make sure there's no pin hole Ina brake line.

Your brake fluid is likely low.

Keith

Bleed the air out of the brake lines.

Garryowen

X2

Thank you guys.

Master cylinder blowing by the rear circuit.

X3

Try bleeding the master cylinder. To do that , have friend help while you bleed the lines coming directly out of the master cylinder. Do one at a time , not together. You should see both are bleeding out the same amount and the same pressure. Just bleed the two lines at the master cylinder not the bunch just below it which is part of the ABS system. If they are inconsistent , you may have a bad master cylinder.

Are you getting any fault codes for the ABS system ?

No fault codes, nope.

That's good. Getting into ABS related problems can make you pull your hair out and shred was your wallet sometimes.
Changing out a master cylinder is much cheaper and easier.

I'll start with that and work my way up. Thanks.

Probably a bad o-ring on the piston. Master cylinders can be rebuilt.

I gotta add a funny story to this thread.

Years ago my roommate at the time was dating this girl. She had a mid-80's Chevy Malibu type car. It had brake problems. She had already had one shadetree mechanic replace the front brake pads. Didn't help. Same mechanic then replaced one of the calipers. Still didn't help. Then she had a different shadetree mechanic replace both of the rear wheel cylinders. Still didn't help. Not the girl's fault as the diagnosis was made by the people that worked on her car.

My roommate asked me to look at the car. A fairly quick diagnosis revealed that the master cylinder was bad and it was only making the back brakes work , fronts had almost no use. So I told her it needed a master cylinder and the part was around $50 at the time. The look on her face was desperation and the thought of spending more money on a problem that didn't get fixed after several tries and the money spent. My roommate backed me up saying if that was the problem , I was probably right. The girl caved in and returned the next day with a new master cylinder. Fairly easy job , got it installed in about 20-30 minutes.

I then give her back her keys and tell her to wear her seatbelt until she got used to how the car was going to stop. In one ear and out the other.......

She came back a couple days later to visit my roommate. Then I got the story.....lol

She had left with the car after I fixed it with another girl. The first real stop they made the driver was so used to pushing down super hard on the brake pedal they completely locked up the brakes. The driver slammed into the steering wheel and the passenger hit the dashboard and faceplanted the windshield. Obviously not wearing their seatbelts. Luckily nobody got hurt too bad. The girl that owned the car thanked me for fixing her brakes correctly unlike the people before that failed.

Moral of the story - Wear your seatbelts.

Psh..........! Brakes just slow ya down.