I need some help with my brakes. The car is a 70 Chevelle w/power brakes, disc front, drum back. Problem is that if I apply any significant pressure to the brake pedal, the rear brakes lock. The problem began after reassembly of the entire braking system. I did replace the rear wheel cylinders, brake booster and rebuilt the master cylinder.

did you install the rod from the brake booster onto the correct hole in the brake pedal?. From my reading many posts on this forum, I am told there are 2 holes on the pedal. one is for power brakes one is for ordinary brakes. If you use the wrong hole with power brakes, I am told you get very sensitive brakes. Now, which hole is correct for you?? Hopefully someone will chime in to tell us, or you can search brake pedal on this forum and I am sure you will read the answer. I will check my files and post back later if I find which hole it is.

Power brakes use the bottom hole on the pedal. Did you use similar compound brake pads and brake shoes?

Andrew

Do you have a proportioning valve and not just a distribution block?? I think if you had a disc/drum combo and a dist block it would do what you are saying.

Originally posted by bhawk:
did you install the rod from the brake booster onto the correct hole in the brake pedal?. From my reading many posts on this forum, I am told there are 2 holes on the pedal. one is for power brakes one is for ordinary brakes. If you use the wrong hole with power brakes, I am told you get very sensitive brakes. Now, which hole is correct for you?? Hopefully someone will chime in to tell us, or you can search brake pedal on this forum and I am sure you will read the answer. I will check my files and post back later if I find which hole it is.

I just came from mpbrakes.com, this is a great site for info on how brakes work and to troubleshoot. The site confirms that power brakes use the bottom hole. Go to that site and read all the info there. It will help. They mention that some master cylinders use a different length rod going to the booster. A rod that is too long will cause the rear brakes to drag, obviously resulting in lock up upon first pushing the pedal. Jack you car in the air and see if they are dragging due to this or due to overtighening the brake drum adjusters. Post more info on what you did exactly also. Was your car originally drum brakes all around and later converted to disc in front? You say you rebuilt the master, did you use the old core or did you buy a replacement unit? Tell us the ultimate fix so this thread can be complete.

Sorry...I haven't had much time to venture into these great suggestions. It appears to me that the number one check will be to ensure I have the rod in the correct position, since the car has the original configuration of front disc and rear drum I think this may be a good possibility. FYI...I have back the rear brakes off, the master cylinder is original (rebuilt) the booster has been changed, all other components are original. I will also visit the brake experts mentioned in your previous reply. Thank you....TH

The hole you use will only change the leverage, so if you use the wrong hole, the brakes will be "touchy". It can't make the rear brakes lock though, so something else must be wrong. I would start by bleeding the brakes thoroughly (especially in the front), and if that didn't work I'd look at how the rear brakes were put together (are the adjusters working alright, did you turn the front/rear shoes right etc?).