1995 Civic Dx=3 stumped ASE Master Mechanics?

Hi there. I have a 1995 Civic Dx with 181K on the odometer. It is a 4 door manual Trans car. We brought the car in due to front end issues. They installed new struts and also noticed my e-brake needed adjusting and adjusted it for me. 2 days after we picked the car up, when I went to stop the pedal went to the floor and the car didn't stop. Immediately after the brakes returned to normal function. Ever since this moment the brakes work most of the time and do this at random.

I installed a new master cylinder myself and still had the same issue. I then took it to my trusty mechanic shop, they bled and bled and bled it before deciding it was a bad master cylinder. The new one they put on didn't fix it either. They have since replaced it 3 times and also replaced the proportioning valve, power booster, power booster vacuum line, rubber brake lines x 4, wheel cylinders, front calipers and for good measure pads and rotors. They did a great deal of this work for free since my bill is now likely greater than the cars value. They brought in 3 mobile Master Mechanics from the corporate office all with ASE Master Mechanic Certs and nobody has determined the culprit.

At this point they advised me to go to the dealer who told me it needed a master cylinder. I had already explained the above but he wasn't interested in entertaining any other ideas until I agreed to another new master cylinder at the cost of $600 something $. At this point it is sitting in the driveway because I am scared to give it to anyone else who may just rack up more $ and not fix it. I can't really back out now either as it has little value without reliable brakes.

Any ideas? Thanks in advance, I am at my whits end with this thing.