I have a 94 Mercury Sable GS, and for some time now I have had air in the brake system. I have tried to bleed the rear brakes with a vacuum pump like the manual that I bought from Auto Zone say'd. I followed the instructions and even had my stepfather who is quite mechanically inclined and knowledgable help me, but air remains in the system and it takes extra effort when pushing the pedal to slow the car down. I have to hit my brake pedal very early and often, and some times to the floor(depending on my speed of the car) just so I can make a safe stop. My stepdad said that I need to have the breaks bled from the master cylinder, but the Chilton's manual I bought says nothing of this. There is not a leak in the system because I have replaced the calipers, and break lines since buying the vehicle so i know it is not a problem with the parts. Can anyone help me with a solution?

I have a '95 Chevy Cavalier w/130,000 miles...lately, the rear passenger side brake will seem to "grab" or "lock" temporarily without stepping on the brake pedal...seems to happen when I'm just starting out to drive in the morning and on cold days...after I've been driving a while, it doesn't do this...there are disc brakes on the front and drum brakes on the rear...usually happens when backing up, but also happens when going forward, also...yesterday, it practically stopped me from moving...almost feels like i backed over something or hit something when it happens...the rear of the car lurches downward when it does this...I have always set the emergency brake firmly when i park...and left the car in gear (standard transmission)...I have changed the front disc brake pads myself, but the rear drum brakes have never been serviced...I hear they are complicated to work on... Any advice greatly appreciated... Sincerely, Matt