I've got a strange problem with a 95 Mitsubishi Montero truck.
The brakes worked fine till one day when the brake pedal decided to become really spongy and also engages very low (close to the firewall).
After checking all the calipers and pads (they're new), bleeding the system, I decided it was a bad master cylinder.
Changed the master cylinder. no effect. Could've been a bad new M/C so I chaned it yet again. no change.
With engine off, the pedal is solid / hard and doesnt drop at all even after constant pressure. Hence I'm assuming there's no air in the system and M/C is good.
As soon as the engine is turned on, the pedal drops almost 5 or so inches. With the engine ON and car running I can get a high pedal if the pedal is pumped. Given I've bled the system thoroughly atleast 8 times by now, bench bled master cylinder as well, could someone kindly tell me what else could be causing this issue ? It does have ABS but the ABS isnt working due to bad ABS sensors. The brakes worked fine for a year even after ABS wasn't working so I'm assuming ABS is not related to this issue... any suggestions would be appreciated.

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