I have a 96 civic with a ZC swap that was never finished right and was still running obd2. Converted it over to obd1 wired in vtec, changed intake manifold & map sensor and got rid of all the cel codes. Now the only issue I have is that my brake vacuum is "weird" at higher rpm it seems like i have no brake issues, at low rpm say when pulling out of a parking spot my brakes go soft and go to the floor. Which mind you this doesn’t happen all the time, it’s completely random but within a day of driving to work and lunch it probably happens 3 times, so it’s fairly common but it’s hard to reproduce. It seems if I apply the brakes in a parking lot to do a 3 point turn its almost always a given that the breaks will be on the floor weak..

Things I have replaced;
1) brake booster
2) master cylinder
3) one way vacuum line to break booster
4) rear wheel cylinders
5) all the pads

I thought the evap solenoid might be causing it so I capped it off on the intake and I still have the same issues, does anyone have any ideas what might be causing it? just found out I’ve got a baby on the way so ya know it inspired me to make the car a little bit safer