Hi this is my first thread im normally search and read but iv been searching
for a thread on brakes failing. iv got a 91 vitara 2dr its got about 40mm suspension lift and 30mm body lift also 30-9.5 muddys and iv been having my brakes fail sometimes and the pedal go`s soft if i push to the floor 1 wheel frotn or back locks up,sometimes the 2 back do i didnt really worry to much but i nearly got into smash so time to do somthing. Another guy i know has the same problem and changed brake boster and did nothin. Any help would be awsome thanks brad

I assume that you've checked for leaks?
Some early models have a Load Sensing Proportioning Valve (LSPV) that has a spring from the valve to the rear diff. This adjusts the brake balance relative to the load. If you haven't taken steps to correct the adjustment of this then your brake balance will be all wrong

If you're not losing any brake fluid (reservoir is pretty much full), this has all the earmarks of a master cylinder gone bad. 

When the booster goes bad, you don't lose brakes or pedal, pedal effort just goes up a ton.  Exact opposite of "soft" pedal.

Rebuild or replace that master cylinder asap.

thanks heaps for that fellas