69 Mach I Brakes - Hard pedal
#1
My brakes have been giving me trouble for the past year. This is driving me nuts, brakes aren't rocket science. Anyone have suggestions for these symptoms?

The brakes work Ok for about the first 1/2 to 3/4" of travel in the pedal. Then it feels as though the pedal hits the end of it's travel and I almost have to stand on the brakes to get the car to come to a stop. I have 15 inches of vacuum at the booster, a new master cylinder (a year ago) and a new booster. All of the brakes bleed Ok, so there doesn't appear to be anything clogging a line or the proportioning valve. The pedal doesn't fade at all - it's rock solid after that first 1/2 to 3/4" of travel.

Can anyone tell me what length push rod you use from the booster to the master cylinder? It came with two - a long one and a short one. I did lots of measuring last summer to determine the right adjustment for the rod and used the long one. I'm assuming that the MC end of the rod should just touch the piston in the master cylinder when the brake pedal is at rest. To that end, I measured the depth into the master cylinder to the piston and adjusted the rod so that it just touched the cylinder.

Any other suggestions of where to look?
Thanks,
Hutch

Some other updates ... I had an inspection mechanic buddy look at it today as well. He just called to say that he pulled drums off the back and verified that, with the car running and the drums off, the shoes expand out normally and the brake pedal has a full range of travel. When he puts the drums back on, the hard spot returns.

Likewise, when he takes the pads out of the front calipers (and inserts a piece of oak so that depressing the pedal doesn't blow out the seal in the caliper cylinders), the brakes seem to work flawlessly and the pedal has normal travel. When he reassembles the front brakes, the hard spot returns.

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#2
It sounds like your power brake booster isn't working correctly. Unrelated to the hard brake pedal, make certain the push isn't too long and not letting the pistons inside the master cylinder to fully return.

The push rod length is measured from the master cylinder mounting surface on the power booster to the end of the rod. That measurement is 0.980" to 0.995" per my 1969 Ford shop manual.

Best Regards,
Mike
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#3
(04-14-2015, 08:25 PM)1969_Mach1 Wrote: It sounds like your power brake booster isn't working correctly. Unrelated to the hard brake pedal, make certain the push isn't too long and not letting the pistons inside the master cylinder to fully return.

The push rod length is measured from the master cylinder mounting surface on the power booster to the end of the rod. That measurement is 0.980" to 0.995" per my 1969 Ford shop manual.

Best Regards,
Mike

Thanks Mike. That's really helpful, because I think that's what I have. I measured independently last fall and then took a photo of the final rod length coming out of the booster. It certainly looks like it just shy of an inch. I've attached the photo.

How about the proportioning valve? It appears that only the rear brakes are attached to the proportioning valve in my car. It doesn't look stock - it has knurled, blue-anodized aluminum adjusting knob on it. When I adjusted it out, the car seems to stop better, but there's still the hard spot. I think the Ford shop manual for the 69 shows the front AND the back going into the proportioning valve. I don't have the book here with me, my car is still with my mechanic buddy, who is still somewhat stumped. Later tonight I'll take a photo of the proportioning valve and post it to see if it matches what some of you other guys have.

Regards,
Hutch

edited to add pictures of the proportioning valve (taken when I put Borgeson steering in)

Notice that the front brake line goes to a solid brass distribution block while the rear lines go to the proportioning valve. Is that correct?


Attached Files Image(s)
           
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#4
That adjustable proportioning valve is definitely not original. It looks like somebody did a drum to disk conversion and not have all the parts. Or tried to improve the braking by adding the adjustable valve. What is that brass fitting to the right of the proportioning valve in the picture? It looks like a Tee fitting for some reason. Why does the booster have those spacers on it where the master cylinder attaches? I have never seen that. But, the push rod length looks correct so that is probably not an issue. Even though it acts like a car with a bad brake booster, I guess before condemning the booster, get your proportioning valve setup correct.

This is not mine, but my original setup looks like this. It mounts the inner fender below the master cylinder. They reproduce them again but they are relatively expensive. I was able to buy a seal kit and rebuild mine.
   
I am not 100% certain but I think the original setup is merely to trigger the brake warning light and provide a residual check valve for the rear drum brakes. The proportioning is designed into the master cylinder and wheel cylinder bore sizes.

Best Regards,
Mike
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#5
Thanks Mike,

The mechanic also asked about that aluminum spacer between the booster and master cylinder. It was on the old booster, and the new master cylinder/booster setup I bought a couple of years ago from Rock Auto had it as well.

That brass fitting is indeed a T. It splits the single line from the MC to go to both front brakes.

I think you're right with the last sentence; the proportioning valve that you provided a picture of looks like a distribution block with a pressure sensor. It's odd that it loops back on itself like that.

I'll investigate the prop valve / dist block further, thanks for the feedback.
Hutch
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