i have a 194 5.7l Blazer. the brakes do not hold when stopped, in drive- the brakes begin to shutter after a few seconds, and no matter how hard I press on the brake pedal, the brakes do not fully hold, and the truck slowly moves forward. I have been to 2 mechanics, and neither know how to repair this problem. I checked for a vacuum leak at the brake booster, but did not detect a leak.


help!!

 

Have you checked the master cylinder fluid level? If it has gotten low, you likely have a leaking line and/or wheel cylinder somewhere on the truck.

If the master cylinder has gotten low at any point and air has been pulled into the system, the system (MC & all lines) will need to be bled.

The master cylinder bleed is VERY important. Any air in the MC will result in a non-existent pedal. I typically like to pull the MC, mount it in a vice, and put on my bleed lines (lines made to thread into the ports on the MC and bent to direct the fluid back into the reservoir), then push the MC plunger in repeatedly until only fluid (no bubbles) come out.

Once the MC is bled, you have to then bleed all of the lines starting at the wheel furthest from the MC and working to the closest (right rear, left rear, right front, and the left front in that order). You must bleed the lines until you get clean fluid without any bubbles in it. While you bleed the lines, you MUST keep the MC topped off.

I would hope that two separate mechanics would have already tried this...

 

master cylinder is approx. 1 yr old, lines have been bled twice, all fluid replaced. front pads and rotors replaced. what else can I have done?

 

When did the problem first start in relation to what parts were installed? If there are no leaks and the lines are free of air, that leaves the master cylinder as the culprit.

Was the master cylinder bled properly way back when it was replaced? New or not, if it isn't bled properly, it will not apply pressure to the calipers or wheel cylinders, period.

If the MC was bled properly, then it likely is faulty.

Even without a functioning brake booster, you will still be able to apply the brakes. You just won't have power boost for a nice smooth, low resistance pedal feel.

 

master cylinder is 1s new, the problem started about 2 months ago, slowly getting worse. I have had 2 different shops bleed the brakes, replaced the fluid. I did new front rotors and pads. how about the rear brake cylinders?


here is the issue: when stopped in drive, foot on brake pedal: if you let off the pedal pressure, and feel the brakes give loose a bit, and you get that "shutter", that is what is happening now, no matter how hard I press down on the brake pedal when stopped. It does not happen instantly, but by 30 seconds+, it then gets worse!