My stepson has an 87 YJ, unstarted - the brakes have good pressure, as soon as he starts it, the peddle goes to the mat, no pressure at all. He has replaced the master cylinder, the power brake booster and bled out the lines. Can't figure out whats wrong! any help would be appreciated.

 

A lot of guys would probably be afraid to answer this question due to liability, because can never seriously be diagnosed without visuals. If you have new components all the way around, a low pedal can be contributed to rear brake adjustment. But there's more to it then that. The parking brake cables can cause serious low pedal issues.

First, check parking brake cables, and make sure they are free. If not they will hold the rear brakes on partially, and cause the wheel cylinder pistons to be farther out then they should be. The arms the cables are connected to in the drums, need to be all the way off the equalizer bar. That's the flat piece of steel that runs from shoe to shoe under the Wheel Cylinder. That bar should be touching both shoes, and the PBrake arm should be just touching it. If the tension is on the Pbrake arm, that's the start of a low pedal. Make sure when you put the drums back on, there's no rust ridge on them. Because if you are adjusting the brakes to that ridge, you're in for a low pedal. If the ridge is too thick, grab new drums. They are usually cheap.

As for the front brakes, if you look at the spindle (where the brakes attach to the axle) and see notches worn into the metal... that's a big issue. The brake pads will get caught in those grooves, and cause serious probs. Low pedal, and brake steer (pulling). Usually, in this case, I modify the pads and throw in some guides from Ford brakes on the slider face. there are clips to put on the pads you can buy from certain parts stores, but they are usually inefficient. Gluck

 

first of all check for the fittings of the brake alignments and the level of brake oil in the brake cylinder and then seek the help of mechanic to overcome the problem....................:):):):):)