My 1990 chevy pickup brake lights are flashing a code of 4 short and 1 long. The brakes on my pickup have never been very good but I haven't noticed any change. The only way I have found to stop them is to unhook the battery a few minutes. My scan tool will not read brake codes. What could be causing the problem? There are 195.000 miles on the pickup. I have changed the master cylinder 2 times. It has a 350 engine and it is 4wd.