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I have a 1992 Toyota 4WD PU with 250000KM on it. The front brakes began to engage on their own as the vehicle ran for about 20 minutes. When the engine was cold, the brakes were not engaged however after about twenty minutes after the engine had been on, the brakes basically locked the front wheels. Is it the master cylinder or brake booster that could be causing this problem?

You may have a problem in hoses, callipers, or master cylinder or all of the before mentioned parts. You need to approach this systematically. When the brakes lock up do the following steps: 1) Loosen the master cylinder housing from the brake booster. Did the brake free up? Yes/No? If they did not free up then it is NOT an adjustment rod problem. (In most cases this is not the problem) 2) If the brakes are still locked up, loosen the lines going to the master cylinder. Did the brakes loosen up? If yes, then there is something wrong with the master cylinder and it needs to be replaced. (If so, you must bench bleed the master cylinder before installing. 3) If the brakes are still locked up! Loosen the bleeder screw on the calliper one wheel at a time. Did this cause the brakes to loosen up? If it did, then the brake lines themselves are collapsed. They should be changed in pairs. 4) If you have done all of these things and the brakes are still locked up. Then you have stuck callipers than need to be replaced. Don't attempt to rebuild them for safety sake, just replace them. Look at the flexible brake lines and make sure they are not cracked or dry rotted. You said you had 250Km on this vehicle. If they are OE they are due to be changed. Now to the condition of the brake fluid! (If it is contaminated with petroleum products, then everything with rubber will have to be replaced. Which means $$$$$$ and this is another article in itself) When you find the offending part(s) make sure that you change ALL of your brake fluid completely. You do this by flushing new brake fluid through the ENTIRE brake system when you bleed the system. You will use about 1 qt of brake fluid to bleed the front and back brakes. CAUTION: Never let the brake pedal travel lower than the normal brake pedal travel while bleeding the system. If you do, you MAY damage the master cylinder. (The piston seals will go into a rusty area of the master cylinder thus damaging the seals) Make sure your rear brakes are properly adjusted and you should be good to go for another 250Km!


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