When you have drum brakes on the rear, badly-misadjusted shoes will cause a low brake pedal that will be overcome by pumping the pedal multiple times. The pedal will go low again after it is released for at least a few seconds which gives the shoes time to retract away from the drums again. You have disc brakes on the rear so we can rule that out.
Rear calipers that have the parking brake built into them will not self-adjust simply from working the brake pedal like all front calipers do. The parking brake has to be exercised a few times to cause them to adjust. Until that is done, the brake pedal can be too low because a rear piston will travel much too far before it contacts the pads and pressure can start to build, but they will not stay there. The pistons will pull back again rather than self-adjust. This is not an issue with your car either because your parking brake is a separate internal drum brake similar to the very trouble-free design Chrysler used for many years.
You have ruled out an external leak, and that would not be intermittent. An internal leak inside the master cylinder can be intermittent like you described. Based on the recent work that was done, my vote is for internal leakage in the master cylinder. There is a trick you can use to make replacing it easier, but this might be a little tricky on your car. Remember when I said to never push the brake pedal more than halfway to the floor to prevent tearing the lip seals in the master cylinder? Well, that does not apply to a rebuilt unit that is less than about a year old because that crud has not developed yet. You must bench-bleed the replacement unit and it is okay to run the pistons all the way in.
Once you are done bench-bleeding the new unit and it is ready to install, loosen the two steel lines on the old master cylinder, unbolt it from the booster and pull it away, then use it as a handle to bend the steel lines up a little so the brake fluid will not run out. One of your lines comes off the top so you are on your own for that one. Just do whatever works. Screw the lines onto the new master cylinder, use it to bend the lines back down, then bolt it to the booster. Now, with those lines still cracked loose, have a helper push the brake pedal down very slowly. It should take about fifteen seconds to run the pedal halfway to the floor. As he does that, you'll see air bubbles pop out by the line nut, at least the lower one. Snug those nuts for both lines, then holler to your helper to quickly release the pedal. Once it is fully-released, loosen the nuts again about a quarter turn, then have the helper slowly push the pedal again. Be sure to tighten the nuts before he releases the pedal, otherwise air will be drawn back in. At most you might have to do this three times until you do not see any more bubbles.
Now, with both lines fully-tightened, have your helper push the pedal again, but still very slowly like before. Use a wrench to tap on the steel lines while he is moving the pedal. Once the pedal is down about halfway or so, have him release the pedal, again, very quickly. Do that a few times and you are done. By pushing the pedal slowly, the few remaining air bubbles in the lines will float back up while brake fluid gets pushed down to the wheels. Tapping on the lines with a wrench will loosen the air bubbles that stick to the lines. By releasing the pedal quickly, the fluid rushing back up will wash the air bubbles into the reservoir. This should work for the front line that comes off the top of the master cylinder. It will be easier to convince those bubbles to travel into the reservoir than to travel all the way down to a caliper.
This trick works well too when you are gravity-bleeding the brakes and the reservoir runs empty, as long as you catch it right away. Another trick if the wheels are still off is to use a flat-blade screwdriver to pry the piston back into one of the calipers. That will push the brake fluid back up rapidly to the reservoir and take any air with it.
Saturday, April 4th, 2015 AT 7:39 PM