Spongy brake only when engine is running.

ONEEYEDJACK1

MEMBER
CHEVROLET TAHOE

Help! I have a 2002 Tahoe 4x4, 129,000 mi, 5.3l engine. First let me say, I am a former mechanic, only from more than 30 years ago, but still do most of my own work. Recently, I put in new pads on all four disk brakes. I replaced the rotors on the rear wheels. Drove for over six months with now problem. Then I suddenly had a spongy brake with the motor running, and have since replaced the master cylinder and the power brake booster, and also had the brakes bled by a local shop, as I understand the ABS has at times caused difficulty in bleeding the brakes. The brake pedal is hard if you pump them up when endine is not running, and it stays firm as long as you hold pressure on it. However, if the engine is running, it will fade to the floor if you continue to hold pressure on the pedal. This is a bit of concern, as I tow a 2000 lb trailer at times, and am concerned about the brakes being there on a quick stop. I have talked to several mechanics, including goodwrench certified, and no one as any further ideas. How about you? Any help would be appreciated. :Roll:

1 Reply

MERLIN2021

MECHANIC
17,250 POSTS

No leaks? Check the bench bleed job on the master, replace if needed, may have got a defective rebuilt!
Also ckeck the calipers for any fluid inside the dust boots. If you have a plug kit, undo the lines at the master, screw in some brass plugs, and pump the pedal half a stroke at a time, the pedal should get to the point you can't get any travel out of it. Now all the air is out, re attach the lines and bleed the lines.
If the master wont pump up it's NG!