I've been trying to fix this 2000 Honda Civic LX 4 door (NO ABS) for a while now.
The issue is that when I turn the car on, there is absolutely no resistance in the brake pedal and it is spongy. For some background, some parts broke and long story short I had to replace segments of brake line, and well air would get in the brakes so it would have to be bled. I have bled the brakes multiple times, and now when I continue to bleed them I don't see any air coming out of the bleeder valves. There is resistance when the car is off, then when I pump the brakes, hold, then turn it on, it loses all resistance and everytime I press down on the brake pedal (when the car is on) there is an audible hiss around the brake pedal (no idea where its coming from, not sure if its a torn diaphragm/seal or if its even a problem). This led me to do a brake booster test but according to how its done its good (brake sinks a couple of inches when turned on). So which leads me to if its not a bad brake booster and the brakes have been bled multiple times this le ds me to a failing master cylinder (pain, which I'm going to order parts and proceed). I'm hoping I answered my own question but any help is greatly appreciated, again I'm not 100% sure it is a failing master cylinder I could be clearly missing something. I'm sure this isn't going to be the last repair on this civic it's originally from the OH/WV area and has a great deal of rust damage on the underside, I'll be sure to post updates on this civic, anyways thank you all.

Hi again, no luck. Replaced the brake booster and master cylinder (and bench bled) and still nothing, I also replaced a segment of brake line (master cylinder secondary line to proportioning valve). Now there is no resistance at all both on and off. There is still the noticeable hissing sound from the brake booster that I've determined is normal as my 2004 Honda CR-V has the same sounds and is functioning. I'm still looking into it but as far as that I have no new leads, I've been tightening the segments of brake lines i've replaced ensuring there is no leakage or fluid from them which would cause air to get sucked into the brakes. I feel like I am missing something but I have been replacing every part there is to replace (other than all the brake lines). Again any suggestions is greatly appreciated.

Update: I believe I found where the hissing may be coming from and where air could be leaking in from into the system. There is a gasket between the brake booster and the master cylinder, when I installed the new parts I did not get a new gasket (it did not come with it) so I just used the old. However after bleeding the system and testing and trying to diagnose, I went back to the master cylinder and took it back off and sure enough there was brake fluid leaking down where it connects to the brake booster. I just ordered a new gasket, it won't come until a few more days. Really hoping this is the cause of all the issues, until then I'll get back when I get another update.

2/8/26 Brought it to a Toyota dealership where a ex master Honda tech now works, they just did the routine inspection and according to them 'found no vacuum leaks'. Now this obviously can't be true, as the brakes where spongy when we picked it up and I was going to see how it was but the brakes just got worse and more spongy. So taking it back to the Toyota dealership. If anybody has had this issue and has been able to diagnose and fix it please reach out, I'll be active until this is fixed and I have an answer.