2001 Isuzu Vehicross

I have a brake problem on my 2001 Isuzu Vehicross that has mechanics at 3 garages stumped.
I bought this SUV in Dec 2013 and drove for 9 months with no noticeable problems. At PA state inspection in Sept 2014 all 4 brakes and rotors were replaced. Since then the brakes slowly started to not engage when the pedal is first pushed. It progressed until they didn’t engage until about an inch from the floor. In Nov 2014, the brakelines were checked for leaks and bled twice. Each time they worked fine for a few days, then started to not engage again. A friend suggested maybe it was not brake fluid in my brakeline. A mechanic assured me they contained brake fluid. Needless to say I did not drive this SUV much over the last several months.
Then in July 2015, another mechanic thought it was the master cylinder so I had that replaced. It did not fix the problem. It has progressed to the point where I can feel they are engaging when pedal is close to the floor, but not very strongly and I travel further & further until I finally stop.
There are no dashboard indicators, nor codes on a diagnostic computer showing a brake problem. I have never seen brake fluid on ground under the vehicle.
Does anyone have any idea what might be causing this brake problem?
Thanks for your help.

I cannot think of anything on a car with 4 wheel disc brake that would cause this except a bad master cylinder. Is the master cylinder getting low on brake fluid when this happens? If it is, could a leaking diaphragm in the brake booster be draining fluid?

If there’s no loss of brake fluid, and the engine seems to run the same as normal, no loss of power, irregular idle, stalling, or hesitation during acceleration, I think if you replaced all four calipers and installed new rotors and pads, and an experience mechanic did the job, that would fix whatever is causing the problem. So that’s one option. But that is probably overkill. You would end up with everything new in the brake system though, so there is a benefit to just going for broke.

hmmm … what would I do if I had this problem? hmm … well, did the mechanic bench bleed the new MC? If not, ask him to remove it and bench bleed it first, before re-installing it. And I guess if my mechanic didn’t bench bleed a newly installed MC, I’d find a new mechanic too.

I’d do some basic checks of the power brake booster. Check that it holds vacuum ok. That is quick to do. If you engine runs ok, I doubt that is the problem though.

Next, I’d re-bleed the brakes, making sure to follow exactly Isuzu’s recommended procedure. On a 2001 you may have ABS and bleeding the brakes with that feature can require an Isuzu scan tool. Some vehicles have a brake accumulator, and that can require special bleeding methods.

After making sure the MC was bench bled, and system bleeding, if that didn’t fix it, I’d replace all the rubber hoses.

Still not working? I’d replace the MC again. With an OEM MC if one is available.

Still a no-go? Replace all the calipers, pads, and rotors.

Still a no-go? Replace all the metal tubing.

Still a no-go? At that point, I’d either junk the vehicle or bring in the experts.