About six months ago I noticed my brake pedal was going to the floorboard, requiring several pumps to get pressure. Right away I thought 'master cylinder' so, I replaced it. Several weeks later there was air in the lines again, so it seemed, so, I bled them again. Then several weeks later I noticed the same thing, this time it pulled to the left when I hit them hard so, I pulled off the right tire and sure enough the right brake had a lot of air it it. Now, every three or four weeks, depending, I have to pull the right wheel and bleed out a lot of air, none in the other wheels, I've done them also. There is no sign of a leakage, can I get air in my brake lines without brakefluid leakage?

C3's are notorious for problems bleeding the air completely out of the system. 

When you installed the new master cylinder, did you bench bleed it first?  Hopefully you didn't mix DOT 3 and DOT 5 brake fluid...that will cause major problems with spongy brakes.

Another potential is what happened with the '78 I owned.  It was very much like the problem you're experiencing.  A complete bleeding of the system, which was followed by a new master cylinder with more bleeding and the pedal still kept getting spongy.  It turned out a piston seal in one caliper was bad and instead of leaking fluid it was sucking in air. 

You might try having that right rear caliper rebuilt or replaced followed by a complete bleeding of the system and see if that corrects your problem.

Thanks Gunslinger, it is good to know there is a similar instance happening to others, I thought it might just be me! I thought there might be a chance of sucking air instead of leaking fluid. I've re-bled 5 times and still I get the same reaction after a couple of weeks, I'll try the piston seal since I have to take the wheel off anyway.

Check the runout of the rotor.

dwright is right about checkin rotor runout.  That is most often the case with a recurring problem like yours.  In theory, the rotors should be trued while riveted on.  If they are no longer riveted, at least mark them so they go back on the same way each time.  As a matter of fact, if they are not riveted, and you can check runout on the car, keep turning the rotor one hole on the hub and see if you find an acceptable location, then paint marks on both.

Good luck.

Get buy/borrow a magnetic base and a dial indicator. Make sure the rotor/hub mating surfaces are clean and free of anything that may jeapordize your reading. Bolt on the rotor,wheel off of course, using a torque wrench to tighten the bolts in a criss-cross pattern then set the indicator on the rotor and slowly spin it. .005 indicated is about the most you would want to read. Any more then you have a runout problem. .000- .003 is what you want. If its more then loosen/clean/remount the rotor. Then check again. Dont want to replace/turn a good rotor. If it is still out I would replace the rotor. It is better to get a new one than have it turned. I never had any luck with the local grease monkeys ( Bubbas relatives) who slap the rotor on a rotor lathe and have it come out right.Ah yup is fine now gime 20 bucks. Ahh huyuk yuk yuk. UMMM Nope!custom762008-02-05 06:38:43