Chap. 4 _63 brakes saga_found an AIM note

[Steve D.]

In the 63 AIM, Section 5 - Sheet 4, I found the following note:

"Pedal must support a 50 lb. load at "A" to a height of not less than 2.50 inches at "B" and have 0.06 to 0.25 in. free travel." ( "B" is shown to be measured from the floor to the top of the pedal ). If this is saying that 2.5 in. is the minimum spec, then my 3 in. measurement would be within the spec. Or am I, in my desparation, misreading the note?

Steve

[John H.]

Re: Chap. 4 _63 brakes saga_found an AIM note

Steve -

You interpreted the note correctly. It was changed in 1965 to a maximum of 2-3/4" of pedal travel; if more than 2-3/4", brakes were to be bled again.

[Steve D.]

Re: Chap. 4 _63 brakes saga_found an AIM note

John

Just to make sure that I have this right: My current situation is that I have a hard pedal at three inches from the floor (measured to the top of the pedal). Since my starting height is 6 inches, my "pedal travel" is also three inches. Does this fit within the 63 spec?

Steve

[John H.]

Re: Chap. 4 _63 brakes saga_found an AIM note

John

Just to make sure that I have this right: My current situation is that I have a hard pedal at three inches from the floor (measured to the top of the pedal). Since my starting height is 6 inches, my "pedal travel" is also three inches. Does this fit within the 63 spec?

Steve

Steve -

Yes, it fits the '63-'64 spec, but not the '65 spec (master cylinder changed for '65).

[Steve D.]

Re: Chap. 4 _63 brakes saga_found an AIM note

John

Thanks. I guess the saga ends with a whimper. It was probably in spec after the first bleeding two weeks ago, but I was looking for a taller pedal.

Steve

[Duke W.]

Re: Chap. 4 _63 brakes saga_found an AIM note

The disk brake system is "constant contact", so theoretically there is just a very small amount of M/C piston movement to build pressure. The only "pedal movement" on disk systems should be pushrod to piston clearance.

Drum brake systems don't operate at constant contact. There has to be some piston movement before the linings contact the drum.

As a consequence, the drum brake system will not have as much "pedal height" as a disk system, assuming both are in good working order with everything within spec.

Duke

[John H.]

Re: Chap. 4 _63 brakes saga_found an AIM note

Steve -

I don't recall if your master cylinder was new or rebuilt or whatever. However, the component in a drum brake master cylinder that minimizes pedal travel is a residual pressure valve; it maintains 10 psi in the hydraulic system after you press on the pedal and release, to overcome the pull-back springs and move the shoes out close to the drum surface and keep them there, to minimize the amount of fluid displacement required to put the shoes in contact with the drums.

If that residual pressure valve is missing or non-functional, you'll have excessive pedal travel, sometimes requiring pumping the pedal once to get any brakes at all.

Those valves aren't used on disc brake cars, as the pads are in constant contact with the rotors.

[Stuart F.]

Re: Chap. 4 _63 brakes saga_found an AIM note

Just to reiterate a bit; when I finished all my brake work last year and adjusted /bled myself nuts, I got it to an acceptable state and let it go. Since they have now been run in near about 600 miles, and gone through a lot of back up self adjustments, they are great! Nice solid pedal, no fade, no pull. Best drum brakes I ever had in my life - bar none.

Stu Fox

[Steve D.]

Re: Chap. 4 _63 brakes saga_found an AIM note

John

The master cylinder was new when installed several months ago. Is there a way to test the function of the residual pressure valve?

I hate to even mention what I found this afternoon, but here goes. Confident in the fact that the pedal was at least in spec, I decided to look at the push rod length adjustment, thinking that I might raise the pedal a bit that way. In doing that , I noticed that the pushrod was entering the MC at an angle and was binding, and this was the reason that the pedal wasn't returning. After consulting the AIM again, I noticed that it showed two holes on the pedal arm, with the pushrod clevis pinned thru the upper hole. For whatever reason, I had pinned the clevis thru the lower hole, hence the angle and the binding. I moved the clevis pin to the upper hole and sure enough the pedal return problem was solved. HOWEVER, now I had 1+ inches of freeplay in the pedal, and the pedal went all the way to the floor. so now I have two problems; excessive freeplay and pedal travel. The push rod is barely screwed into the clevis, so it is at the longest dimension possible for this particular rod. Is it possible to get a rod that is a bit longer? The pedal going to the floor caught me off guard. The only scenario I can come up with is that the previous 3 in. pedal was not the result of hydraulic pressure, but was the result of the pushrod binding enough to end the pedal travel.

I started back on the bleeding effort. I isolated the master cylinder and got a full pedal again (after the 1+ in. freplay). Reconnected the MC, blanked the rear and right front to isolate the left front, bled the left front and got almost ,but not quite , the same pedal as with the isolated MC. This is similar to the result I got before. I then reconnected the lines to the right front and to the rear. I hooked up my power bleeder and shut down for the night. Tommorrow evening I will start with the right rear, and bleed in the normal sequence. They say that a sign of mental illness is repeating activities and expecting different results. Well...

I appreciate your comments, and I swear that I'm not making this stuff up.

Steve

[John H.]

Re: Chap. 4 _63 brakes saga_found an AIM note

I appreciate your comments, and I swear that I'm not making this stuff up.

Steve

Steve -

You're not the first to mis-position the clevis, and probably won't be the last; common error. Manual brakes used the top hole, power brakes used the bottom hole. All '63-'67's with manual brakes used the same pedal pushrod (#3819077), but I don't have the dimensions.