1977 Jeep CJ7 V8 Four Wheel Drive Automatic 131000 miles
I recently installed a used power brake booster and master (MC) cylinder from a Wrangler. My CJ7 was previously a manual brake nightmare. I bench bled the MC and then started bleeding each brake (RR, LR, RF, LF). I have now spent 14 hours and 96 ounces of DOT 3 bleeding the brakes to no avail. The pedal just will not stiffen up. I checked the vacuum on the brake booster and it is fine (just wanted to make sure I didn't have a problem there). I checked the distance the plunger can travel and it seems to be the maximum that the MC will allow. While trouble shooting, I noticed the following. There are two holes in each portion of the MC; the front one appears to offer a view of the MC piston, the rear one is a feed hole for the fluid (I assume). The rear hole of the rear portion (to the front brakes) emits bubbles. It only emits air bubbles when either the pedal is not depressed or is depressed all the way. The bubbles flow freely for a few seconds and then stop with a bubble formed in the neck of the hole. Why does this thing emit bubbles? Where is the air coming from? There are no fluid leaks in the entire system. I bled 24 ounces of fluid through just that portion of the MC with a vaccuum bleeder and the old push the pedal routine (and that's after bench bleeding the MC). I got air out but there were always more bubbles once the pedal was depressed or retracted. What gives? Are the MC seals bad? Please help.
Since no one answered my question, I'll answer it myself. It was bad seals in the old MC as suspected. When I removed the MC from the booster, fluid poured out (that shouldn't happen). The rearmost seal was so bad, it was letting fluid leak one way and air the other way. Installed new MC, finished bleeding the brakes, and drove it around. Everything worked great. I think I need an adjustable proportioning valve though.