I have a 1992 Civic VX, ever since I bought it the brake pedal had pressure loss. There was a leak in one of the lines so I replaced the lines and hoses, still had pressure loss though it was a little better. I ended up replacing the calipers, wheel cylinders, pads, rotors, drums, and shoes but the pedal still goes down. So I figured I’d replace the booster and master cylinder, which I just finished doing (bench bled the master cylinder), and welp, the pedal still goes down… there is no brake fluid leaks anywhere, and I’ve bled the brakes multiple times. I’d say more than a gallon of brake fluid has gone through the system. The only thing I could possibly imagine causing this issue is a small vacuum leak causing my booster to lose pressure, but the pedal will go down with the car off as well. Anybody know anything I don’t?

 

Bleed the system again. You’re not doing something right

 

Make sure the brake proportioning valve isn't leaking. Easy thing to overlook since it's on the firewall. If the pedal is spongy, then air must be in the system. In that case, I would try bleeding with a vacuum pump from the reservoir. That has worked for me before to get trapped air out of the master, when caliper bleeding didn't do the trick. If the pedal just flat out sinks to the floor when pressed, you gotta have a leak somewhere. If that's the case, you need to check every inch of the hydraulic lines and look for drips on the ground, where brake fluid could be falling from.