buckle up boys this is gonna be a long one. I have a 1996 USDM honda civic LX sedan 5 speed non abs. just a solid daily driver. I had owned the car for about 3 months and decided to disc swap it. I had a set of 94 civic ex abs trailing arms with BLOX spherical bushings and extended studs so i threw them on at work. I had sold one of the calipers to my friend so i purchased one caliper from rockauto to replace it. after I installed the disc conversion the problems started. First off the pedal pressure was not where it should be and would sink at stop lights so i suspected the master cylinder and replaced it with a rockauto stock LX non abs unit. car was fine for a while and then I started having front brake dragging issues. so bad in fact that I couldnt get the car out of my driveway. I ended up fixing the issue by messing with the brake booster clevice pin but the heat from the drag was enough to put the final nail in my 250k original front brake calipers coffin. replaced pads cut rotors and got 2 brand new raybestos front calipers for stock LX front knuckles with 9.5" rotors. shortly after that I was having an issue of a sticking rear E brake on the passenger side which is the caliper I replaced before installing the disc conversion. Replaced that caliper with a NAPA unit. Now here's where it gets weird. when I went to bleed the brakes after work with a co worker it seemed to go fine however the pedal pressure was much worse than before and when the car was on air would become present in the system. bled the brakes with the car running for around an hour with no luck. found the master cylinder leaking AGAIN when I got home so i replaced with an EM1 SI model master on the stock booster. the next day at work I decided to try and bleed the brakes again. same issue. Called a friend and he told me to check the left rear caliper and when compressing it it would twist in and compress all the way but the piston would spin for as long as you twist it and would not stop spinning while compressing the caliper. I have since ordered a new caliper and it should be here tomorrow (4/2/21). yesterday (3/31/21) I checked my master cylinder since my brake light was on on the way to work and I had lost 1/2 of the master cylinder fluid. checked for leaks and found the issue of a leaking banjo on the right rear, replaced banjo and washers and tried to pressure bleed this time. SAME ISSUE. the brake system would bleed just fine but as soon as you turn the car on more air is introduced to the system. I have decided at this point to return the caliper on the right rear since that is the point when the issues started. I plan on replacing both calipers at the same time and re pressure bleeding the system again. Still doesn't really explain the air being introduced into the lines. I guess the TL:DR version of this is HAS ANYONE ELSE HAD THIS PROBLEM OF AIR GETTING INTO THE SYSTEM WITH THE ENGINE RUNNING PLZ HALP. please reply with any questions and I will answer them to the best of my ability.

 

Brake booster is vacuum assist. Wondering if the diaphragm is bad if the engine can directly influence the master cylinder.

The only other thing that comes to mind is pin hole(s) in the lines. Not sure if you can pressurize the lines without the master and see if it holds pressure.

Lastly, not sure the behavior of a faulty proportion valve but if I recall there are seals between the two halves.

Wild ride, sorry to hear about it.

 

while all of these suggestions do have merit and i will check them over just to be sure i dontthink it would be a bad booster because if a booster is ripped the pedal feels rock hard like the car is off. if that were the case the vacuum from the booster would have to somehow make it past the seals in the master cylinder. my brake pedal pumps up fine with the engine off and then drops when the engine is started which leads me to believe the booster is good however a vacuum gauge and a hand pump will tell the truth. for the prop valve issue the ek valves seem to be a solid piece unlike the eg or ef valves which i have had leak before. once again definitely not out of the question but unlikely. I think on Saturday i will leave the system pressurized from lunch time till 5pm and see if there is any fluid loss between then. i appreciate your reply and having a second opinion always makes solving issues easier.