Brakes locking up for no reason

[swathdiver]

50 minutes ago, Victor Amaro said:

So my chevy tahoe all 4 brakes are trying to lock up we changed the front to calibers and it still doing some thang

Well, you might still need to change the rear calipers, the new front ones could be bad or you have a problem with the ABS pump.  You really need a bi-directional scan tool to properly diagnose this so as not to waste money throwing parts at it.

[rooftop903]

On 11/17/2012 at 11:36 AM, Doug_Scott said:

Does the truck have rear disc or drum brakes? If drum brakes, then the compensating port in master cylinder is blocked. If disc, then you need to look at the flex hoses to see if they are breaking down internally creating little flapper valves that restrict the brake fluid from flowing back when brakes are released. Try opening the bleeder screw when the wheels are locked up again and see if releasing the fluid releases the brakes. If it does, then get the brakes to lock up again, but this time loosen the brake line at the master for the rear brakes and see if the brakes release instantly. If they don't, then you need to move down the line to the next component and loosen the line there. Keep doing this until you get the brakes to release. The part that is directly upstream from the connection you loosened is where the problem is.

 

If drum brakes, loosen the line at master cyl and see if it releases the brakes. If it does, then it is either the master cylinder or the brake rod from the pedal to booster, or the rod between the booster and master being too long for some reason.

doug im having the same issue with my 96 but my rear is drum and its only my front brakes staying compressed. any ideas? iv already replaced one caliper and flex hose but they both are still doing it.

[Doug_Scott]

On 7/29/2021 at 7:32 PM, rooftop903 said:

doug im having the same issue with my 96 but my rear is drum and its only my front brakes staying compressed. any ideas? iv already replaced one caliper and flex hose but they both are still doing it.

The more I read my original post the more I think I have it backwards.  The compensating port is for the disk brakes, not the drum brakes.  The drum brakes use a residual check valve to maintain light pressure on the cups inside the wheel cylinders, and that valve also will prevent the line pressure from building from heat.

 

Does loosening either caliper bleeder screw resolve the issue right away?  A quick temporary test is to remove master cylinder from booster (do not loosen lines) and place two flat washers between the booster and the master cylinder.  This will space the master away from the booster by the thickness of those two washers.  Take truck for drive and see if problem goes away.  If it does, then either the master has a plugged or blocked port on the front brake section not allowing the brake fluid to expand into the reservoir as it gets hot, or the connecting rod between the brake pedal and the piston in the master cylinder is too long.  Given the age of the truck, take a good look at the firewall where the brake booster mounts for excessive rust or any cracking.