68 brake issues

[Russ S.]

I replaced my master cylinder (bench bled it) and re-built each caliper (new pistons and seals). Now when I try to bleed the brakes the front calipers bleed fine but then lock up when I apply the peddle. When I release the peddle the pressure in the front calipers remains high. The rear calipers don't bleed much and I don't think I am getting much pressure in the rear lines. I know this is not rocket science but I don't understand why there is pressure in the front lines. Could the pressure sensor cause this problem?

[Gary B.]

Re: 68 brake issues

brake pedal rod, that fastens to the brake pedal and goes out to the master cylinder, is to long and is not letting the master cylinder return all the way.
Shorten the rod a few turns and try it again. Might take a few try's to get it right.
Gary

[John C.]

Re: 68 brake issues

I replaced my master cylinder (bench bled it) and re-built each caliper (new pistons and seals). Now when I try to bleed the brakes the front calipers bleed fine but then lock up when I apply the peddle. When I release the peddle the pressure in the front calipers remains high. The rear calipers don't bleed much and I don't think I am getting much pressure in the rear lines. I know this is not rocket science but I don't understand why there is pressure in the front lines. Could the pressure sensor cause this problem?

I think I can shed some light on this one. My car had very similar issues. There is a fluid return port inside the master cylinder. If this tiny hole is stopped up, fluid can't return to the master cylinder from the lines going to the caliper. What's worse is if driving the car, once the caliper heats up and the fluid expands, it can't return and guess what, it pushes the piston further out and will eventually lock up the brakes. Once this happens, your only option for moving the car is to either ket it cool, or, crack the bleeder at the caliper and bleed off a little fluid to release the piston. The fix, the master cylinder will need to be cleaned really well, blown out with compressed air or sonic cleaned. To verify the return port is open, you should be able to push the piston back into the caliper and see the fluid level in the MC rise. JC

[Stephen L.]

Re: 68 brake issues

Another thing to check if the rod length is correct would be to make sure there is no check valve in the ports where the lines connect. The check valves are used with drum brakes but NOT with disc brakes. Having replaced the master cylinder these may be present. Look at the directions that came with the M/C to see how to extract the check valves.

[Russ S.]

Re: 68 brake issues

I checked the rod adjustment and that seemed to work! Thanks Gary. I thought I had it adjusted correct but it must be really touchy. The master was just restored by White Post so that works great. No check valves, but thanks for the suggestion.