Got a brake issue i am hoping someone can help me with ? 66 el camino, just had a full chassis up rebuild over the last 5 years, including fitting an aftermarket disc brake conversion, car all finished, done maybe 50 miles of short test drives - all good, car loaded up and about to head to my first car show, pull out of the driveway and both front brakes jam on - locked solid - had to drop the hard line off the master cylnder to release the pressure so i could get it back in the garage. I am thinking dodgey master cylinder but open to ideas ???? Anything ??
Sorry should of mentioned, disc brake conversion is on the front, still running the original drum brakes on rear, disc conversion kit included a proportioning valve which mounts off the master cylinder.
Th Thanks Tango, would that indicate the pushrod is too long ? I remember that the pushrod was missing from the kit and the supplier sent me one afterwards. It seemed to fit up ok but maybe its wrong ?
How were things when you bled the system ? Meaning did the pedal travel as expected ? Just asking so as to narrow things down. It's either this or the master by how you described the way you released the brakes.
system bled up really well and in the first 50 or so miles the brakes felt great - was really pleased with the conversion. then it all went bad...... (note: the 50 miles was over a period of days - not in one hit)
Doesn't seem to be a heat issue that happen's sometimes. I'd take apart the master. Possibly there's some type foreign material that's worked it's way into the bore and wedged it from returning. But first do this. Pull your dust boot back to see if the piston has returned against the retaining clip and that the rod is where it suppose to be. Back to the rod issue. Did you go manual or power brake. If you went manual and your master has a shallow rod hole. Your rod may have dropped out and is preventing the piston from returning. Again, peel back the boot and see.
Yup, sounds like the classic piston not returning all the way problem. Follow all of the above advice.
You should be able to work the pedal by hand and watch the push rod to see if you have the necessary slack before it pushes on the piston to let the piston come all the way back to the stop. It doesn't have to move much but you should feel it move and then push against the piston. If you can pull the boot back you should be able to watch it move. I've never seen one drop down and catch the edge of the piston holding it in and pressure on the brakes but that would be something to check when you first stick your head under the dash. The disk brakes won't return the way the drum brakes do because they don't have springs to return the pads. I've seen a couple of cars that had drum brake master cylinders on them that were converted to disk front brakes that had similar problems because they didn't pull the check valve out from behind the seat for the front brake line in the master cylinder. That was in the mid 70's when some guys would swap Monte Carlo disk brakes to the Chevelles and not swap the master cylinder or the rest of the disk brake pieces over.
The pushrod is a fraction of an inch to long. As soon as you hit the brake a few times it warms up and....... Presto.
If you find the pedal push rod has clearance; check that the output rod of the booster has clearance to the master. Found those out of spec on aftermarket stuff; and the directions don't always mention it. Easy way to see; loosen master mounting bolts and slip in a couple body shims. If it cures the hang-up; pull it and adjust the booster rod.
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