Brake pedal going to the floor and staying down

OXCARTMAN AND WIFE

MEMBER
1999 CHEVROLET SUBURBAN
5.7L
V8
4WD
AUTOMATIC
170,000 MILES

We replaced the master cylinder due to internal leak on old one. The brakes were very soft and no fluid in reservoir. After we checked lines and wheels and saw that they were not wet, so put on new master cylinder. Did bench bleed before installed and bled at wheels. Now when you push the brake pedal it goes to the floor and stays there will not come back up. You cannot pull it up. It will only come up if you turn off the vehicle or unhook the vacuum line on the brake booster. The brake booster has strong vacuum. What could be causing the brake to stay down?

1 Reply

STEVE W.

MECHANIC
13,462 POSTS

Well the description sounds like the master cylinder you have is the wrong one. There are a couple different ones out there and they use different length apply rods from the booster. Using the wrong one lets the booster apply valving stay on and it applies vacuum regardless of the pedal position. It also sounds like you have the typical GM brake issue where you need to bleed the brakes two to three times to actually get all of the air out of the system, especially the ABS unit. I would suspect there is air in the lines. These guides can help you fix it.

https://youtu.be/w7gUsj2us0U

and

https://youtu.be/WDxvEQrMkBg

and

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please run down these guides and report back.