Sweptline.ORG

Update: Looks like I had them adjusted to conservatively! Spreading the shoes out returned the pedal to the proper position, and it now rolls and stops in the driveway. Thanks, all!

Bucky

earlymopar wrote: Simple. If the push rod has no free play it can and will hold the piston partially open which causes air to bleed out the ports.
- EM

I was actually thinking too much free play in the adjustable push rod would
add a bit to the pedal travel. I would think if the piston were held
partially open and there was ample fluid in the reservoir, it would tend to
push the shoes outward. I had a situation like that with my '61 D200 when
I first put it on the road. Once I backed off the length of the push rod to
where you could just jiggle it slightly..... the issue went away. Too
much free play in a truck without a pedal return spring.... and the brake
lights would likely come on as well....
John

PwrWgnDrvr wrote:You're missing something on how these systems work, because that's not possible.

Well, sorry but this happens often. Just try adjusting your free play out entirely (as many try to do) and see what happens. This moves the piston slightly and therefore the location of the O-rings which in-turn causes the bore to be open to outside air. If you get to this point and pull a vacuum or use blow through you will hear the air leak!

earlymopar wrote:

PwrWgnDrvr wrote:You're missing something on how these systems work, because that's not possible.

Well, sorry but this happens often. Just try adjusting your free play out entirely (as many try to do) and see what happens. This moves the piston slightly and therefore the location of the O-rings which in-turn causes the bore to be open to outside air. If you get to this point and pull a vacuum or use blow through you will hear the air leak!

However, I did in my haste mis-state the air loss cause. It does not happen with the pedal stroke but in reality with the issues I mentioned above, the pedal is already partially stroked due to the push-rod being improperly adjusted.

- EM

Well, first of all they aren't O rings on the piston, they're cup type seals.

Push rod adjustment DOES NOT let air into the system! The bore is NEVER open to air if the reservoir is full. If you're blowing air with no fluid in the reservoir, of course u will hear leakage.

Read the service manual.
(It also does not say to put a brick under the pedal.)

PwrWgnDrvr wrote: Read the service manual.
(It also does not say to put a brick under the pedal.)

True.

PwrWgnDrvr wrote:Well, first of all they aren't O rings on the piston, they're cup type seals.

Push rod adjustment DOES NOT let air into the system! The bore is NEVER open to air if the reservoir is full. If you're blowing air with no fluid in the reservoir, of course u will hear leakage.

Read the service manual.
(It also does not say to put a brick under the pedal.)

I used O-ring generically speaking. It is a seal, period. Other than that, the condition I've been speaking to does not happen in normal pedal stroking of the M/C. It only happens at a static M/C (foot off of the pedal) with the push rod adjusted to result in zero free play and then you apply either vacuum to the brake line or pressure to the M/C.

If u suck all the fluid out with a vacuum, or blow it all out with pressure, of course there will be air in the lines. But that won't happen merely by
adjusting the push rod incorrectly. This applies no matter whether the master is a single or dual. There simply is no open port to air at any point in the piston stroke when there is fluid in the reservoir(s). If there were, fluid would leak out every time the brakes were actuated!
Furthermore, if a push rod adjustment were so critical as to allowing air into the system, and thus an imminent failure of the brakes, the service manual would have a detailed section on proper adjustment! But, there is no mention of it.

Are you doing this pressure/vac test on an empty master? And why?

Have you tried one of the "guns" that pushes the fluid from the bottom back to the reservoir?

Phoenix Systems Brake Bleeders

There are cheaper versions but here is a link:

https://www.summitracing.com/parts/psl-2104-b/overview/

Several sources mention this symptom and the cause as I explained earlier.

Apples and Oranges.
That is not a diagram of a sweptline era master cyl! They are not made with those components, nor those ports. As I stated above, that condition CANNOT happen on a swept.
(note the DISK brake diagram at the wheels - obviously not from a swept era manual)