Land Rover Repair: 98 Disco brake gremlins, blown hose, careful measurements

Question
John,

I bought and refurbished a '98 Discovery late February/early March and had used as a daily driver of 250+ miles a week up until last week, when I whimsically decided to drive one of my collector cars to work on Friday (5/11). That evening, I took the Disco out to dinner and within 3-4 miles of leaving home, lost braking response - much as you would with a blown hose or caliper - not total failure, but standing on the floor to slow only modestly to a gradual stop. Luckily, I was moving in slow, casual traffic and was able to handle the situation and get home. A few hands-on tests that evening and I diagnosed a bad master cylinder, which I swapped out the next day. On the first drive afterwards, about 3 miles from home, I began losing speed until I had to roll to a stop on a side street and the front brakes were fully applied, pedal was hard as a rock. I determined what line I believed to be feeding the front wheels circuit of the ABS and let off some pressure, enabling returning the vehicle home. Thinking I had and ABS problem perhaps as well, I swapped in another ABS pump, bled the entire system, and once again used the vehicle all week until this locking of the front brakes occurred again last Saturday (5/19) same quick release of pressure, then once home, I removed the master cylinder and took careful measurements of the plunger and dialed in the push pin in the booster until its resting length corresponded to the resting depth of the master plunger. So, was the pin always slightly applying the brakes otherwise? If so, why was I able to commute 200+ miles between incidents? All seems well now, and I admit the pedal did seem "higher" after the master cylinder replacement, but feels more natural now. Is that a common problem/fix?

Thanks,
Michael

Answer
If the plunger is too far out in a master cylinder it will not fully uncover the port to release pressure, which can result in a gradual buildup of pressure. Also, in some cars this does not happen till things heat up and expand which may explain why you didn't see it for a while.  If that proved to be the diagnosis I congratulate you on finding it.  That is a rare thing indeed on modern cars.  We used to see that when we rebuilt cylinders long ago