[SoundGuy66]

Hey all,

I've got a brake problem on my 66 straight six that's got me confused. The problem is that the pedal slowly sags to the floor, but only when light-middle pressure is applied after braking but before being fully released.

This generally happens when I am sitting in traffic. I will brake to a stop, no problems, and then if I do not maintain full pressure on the pedal, I will start to lose pressure. This results in a sort of pumping action, with each stroke getting a little closer to the floor. Releasing the pedal all the way will then permit me to brake normally until the cycle repeats itself.

I have bled all the lines, and I did not notice any significant amount of air.

I really have no idea what is causing this. Could I have a bad wheel cylinder? I have not yet readjusted the brakes, could the left and right brakes being out of sync cause this?

As far as I can see, I am not losing any brake fluid, and this came on suddenly, when I started to drive again about 2 hours after completing a 1 hour drive at freeway speeds. The only thing that may have affected the system during the drive was an episode of rather abrupt braking when traffic suddenly slowed dramatically...

Anyways, I'm at my wit's end. I would appreciate your wisdom-,
-Chris

P.S. I have 4 wheel drums powered by a granada master cylinder. It has a power booster, could that somehow be going bad?

[Trickpony]

Master cylinder is bad or a leaking wheel cylinder...

[SoundGuy66]

So now for the real question. How do I tell which it is? Is it common for master cylinders to go out?

Thanks,
-Chris

[Trickpony]

Yes, and I assume you have drum brakes on all 4 corners, Look on the backplate
to see if it's wet.(where the adjusting holes are).

[Trickpony]

on Your 66 you should have the old mason jar type cap.
Also if the wheel cylinders are the originals I would think of replacing them as well
because a new master cylinder may put more pressure on them and cause a failure...

[mot250]

My 66 master cylinder was so bad once that when stopped long enough, I had to pull the brake pedal back up with my toe to keep from rolling.

If you are not losing brake fluid, it is probably the master cylinder. If you are replacing the master cylinder anyway, I'd go with a 67 or newer dual well master cylinder for safety. I converted mine 10 years ago and just used a standard NAPA replacement dual well master cylinder for a '67 mustang with 4 wheel drum (non-power) brakes. It was a fairly straight forward swap.

[Mexstang]

Adjust your parking brake. Make sure you are bleeding it correctly. Use a buddy. If that doesn't work, replace your master cylinder and bleed correctly.

[Twister]

Originally Posted by Mexstang

Adjust your parking brake. Make sure you are bleeding it correctly.

um...the parking brake is only a cable....

[Cannoball888]

Originally Posted by Twister

um...the parking brake is only a cable....

That reminds me. I need to check my turn signal fluid.