I have a 1998 Chevrolet C2500, I was having brake pedal issues (Pedal would not engage until almost to the floor) so I replaced master cylinder and brake booster. After replacing those the brakes now lock up every few days. The first time the brakes started locking up I checked the master cylinder and saw I got a faulty one so I replaced that again, after replacing I noticed there was no fluid going to the rear brakes so I replaced the abs module after tracking where the fluid stops. So far master cylinder has been replaced twice, brake booster and ABS module is replaced and brakes have been flushed 4 times. Brakes were still locking up so I replaced the booster and master cylinder…….again. (Incorrect bore size on first booster) Found that the brake switch was not operating correctly so tried to replace that and bleed brakes again. Now I have replaced 3 master cylinders, 2 brake boosters, an ABS module, brake switch, and flushed system 6 times and brakes are still locking up. I have checked calipers and all are operating as should, I am at a loss at this point and am hoping anyone has some ideas on what to do next. This issue only started after replacing the booster and master cylinder the first time.

 

Check the pushrod adjustment between the booster and the new master cylinder. If it's too long, it will cause the brakes to lock up by applying constant pressure. It's a very common mistake after replacement. You need to measure and set the correct clearance.