soft brakes - why?

[Greg Reimer 7376]

This won't affect the race car, but the tow vehicle. A guy at work replaced the front pads, turned the rotors, packed the bearings,and replaced the master cylinder on a 98 Suburban with around 50,000 miles on it, and had a problem getting a pedal.Before he was done, the yellow ABS light stayed on,as well as the red brake light on the dash. He had some pedal, but it was soft and low. Trying to bleed it with a SCAN tool didn't help much,even though he followed book proceedure verbatum. I got an idea. It had been rainy earlier, and the street a mile from the shop had some dirt washed out into it, so we took off on a road test.As soon as I got to the wet dirty spot in the street, I slammed on the brakes hard when we got into it. The ABS activated, the pedal vibrated like it was supposed to,the ABS controller made that funky noise it makes,and the light flickered. The pedal got noticibly firmer, and so we did it a couple more times. At the end of about three brake system ABS assisted stops, no lights, firm pedal, brake system normal. We came to the conclusion that the ABS solenoids in the controller must accumulate a rather large air bubble in one or more of the solenoids,giving a soft pedal.Activating the solenoid through a panic stop or two purges the bubble and restores the brake system performance. I also have noticed that vehicles without a noticible brake system issue get a firmer pedal after activating the ABS that way, too. It apparently needs to occasionally be activated.