Power Brake Booster

[Charles Talbert]

Charles and all,
I encountered a mysterious problem the other day with my '52 civi power wagon.
A month or two ago I had a problem with the brakes gradually "locking up" after I drove it for awhile (with a few brake applications). I discovered my push rod length adjustment to the master cylinder was not right. I adjusted this to give the brakes more free pedal travel before actuation and it seemed to help. Several short trips around the farm then showed no problems and the brakes worked fine. The free travel of the master cylinder push rod is about 3/4-1 inch now.
The other day I drove it, however, and the same problem occurred toward the end. After sitting still for 10 minutes though , the brakes "let off".
What alerted me to the problem was two things. FIrst the brake lights stayed "on" without pushing on the pedal, and of course I could not push the truck to roll with the brakes on.
Could the rebuilt Midland booster I have plumbed in the system be part of this problem too? Or is it still an issue with the master cylinder in some way?

This is sounding more like a booster issue. Pedal free travel is not an issue as long as you have some, a very minimal amount is just as good as an inch. The issue is to have just enough free travel so the M/C piston fully returns with no pressure being applied when the pedal is released. Be sure your pedal return spring is holding the pedal all the way to the top.

Sounds like the booster may not be venting properly, if atmospheric pressure can't flow freely into and out of the booster chamber, pressure will not be released. This is also dependent on the large piston return spring doing its job. If it isn't venting freely, the spring is not strong enough to overcome the trapped air. Trapped air sounds like the culprit here since the pressure leaked off within a few minutes. You might try it with a full open vent tube to test the action.

[PWFFnut]

brake booster

Thank you Charles you seem to always have the right answer. I will have My parts truck in the workshop soon & strip it of parts over the summer while I have good weather & don't mind having the 56 outside.

[Doc Dave]

This is sounding more like a booster issue. Pedal free travel is not an issue as long as you have some, a very minimal amount is just as good as an inch. The issue is to have just enough free travel so the M/C piston fully returns with no pressure being applied when the pedal is released. Be sure your pedal return spring is holding the pedal all the way to the top.

Sounds like the booster may not be venting properly, if atmospheric pressure can't flow freely into and out of the booster chamber, pressure will not be released. This is also dependent on the large piston return spring doing its job. If it isn't venting freely, the spring is not strong enough to overcome the trapped air. Trapped air sounds like the culprit here since the pressure leaked off within a few minutes. You might try it with a full open vent tube to test the action.

Charles, to do the test you mention, do I simply take the little air filter off the vent tube end? The air filter is a vintage fram, the shape of a small can designed for boosters.
Then I assume it would prevent the locking up effect if there was some sort of air restriction from the filter?

[Charles Talbert]

Charles, to do the test you mention, do I simply take the little air filter off the vent tube end? The air filter is a vintage fram, the shape of a small can designed for boosters.
Then I assume it would prevent the locking up effect if there was some sort of air restriction from the filter?

That's what I would do for starters.

[Charles Talbert]

Thank you Charles you seem to always have the right answer. I will have My parts truck in the workshop soon & strip it of parts over the summer while I have good weather & don't mind having the 56 outside.

Lets not get too sure just yet, I'm simply basing my reply on offered info. I hope it is the right answer for the issue, as it will be a simple fix if it is. If this doesn't cure it, then we'll have to look deeper.