Check and see if you are losing brake fluid.
First check the fluid in the resevoir and see if it's low.
If it is have look at the inside of each wheel and you should find one that is wet check the hoses,pipes and around the master cylinder and brake booster area.
The fluid can,t dissappear,however it is normal for it to
vary between full and min but not to drop below min to empty.
If the level is ok,the fluid might have boiled.
put fresh fluid in and bleed the system.
You can also try to adjust the brakes.
Another test you can do is to to clamp the brake hoses
at each caliper hose and the brke hose over the diff and see if that gives you full pedal pressure.
You should be able to isolate the problem with one of these test.
Mark
October 2011