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1974 620 Power Brake Flutter - feels like ABS

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This is a new symptom that's started after driving truck for about a year and 8k miles:


If I am going at least 15-20 mph, and I quickly stab and hold the brakes for a very slight braking effort, the pedal will pulsate like ABS while providing the expected slight braking effort.  I can clearly hear "air pumping in and out" when this happens - it is clearly audible in the cab and also vibrates everything because it is also a braking force oscillation too.  


This flutter can be maintained indefinitely going down a hill if the precise amount of pedal pressure is established - and increase and the flutter stops and the brakes work as expected.


Crazy Engineering Theory:  a kind of mechanical-servo instability (oscillation).  The culprit could be air in he lines which acts like a spring.  Since this only happens when I jab the pedal, the jab would kick the mass of the system off it's equilibrium point and you'd get an oscillation.  The PBU assist would then cycle Yes, No, Yes, No...  Slow application of pedal usually does not cause this flutter.  As far as air goes, the brakes don't pump up which indicates there isn't any air in the system.


Some History may help identify what is happening:


TRUCK SAT:  for 10-20 year.




The PBU (power brake unit) had a sticky check valve that resulting in the brakes not be released by the PBU - you pushed the pedal and when you released the pedal, it remained at its depressed position.  Then with some kicking, pumping, etc. I would hear a "click" and the pedal would return. 


At first, I thought something was hanging up in the linkage.  I had the booster out a couple of times and cycled it on the bench with an external vacuum source.  It seemed to work just fine.


I don't remember how at some point I figured out it was a check valve (I confirmed the existence of his check valve by studying the PBU operation).  So I removed the boot and cover from the rear of the unit and spared some dry teflon garage door lube into the booster.  That seemed to make it work OK.




Every PB I've driven has a spongy feel - like there is a balloon between your foot and the pedal.  This unit has a barely detectable sponge at times - it feels like a manual brake pedal.  But it definitely has power.  Again, bad memory, I seem to recall when I first started driving after fixing up, there was more sponge.




Apply brake and hold with foot

Engine off

10-15 seconds later, pedal pushes foot back.

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If the vibration slows with vehicle speed I would suspect an out of round drum or perhaps damaged shoe or missing return spring. Maybe leading and trailing shoes are reversed on one or more wheels.


If vibration is constant maybe the booster.

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The drums are not of round - there is absolutely no pulsation at any other than a "stabbed" very light brake apply.  (and only if stabbed - slow apply and no "flutter.")


Plus, the pulsation frequency is fixed and not a function of wheel rpm.  60 mph or 20, same pulse frequency - it's really feels like a pulse modulation (why I said ABS.)


It's very weird.  One local mechanic rode with me and never saw anything like it.  A couple other mechanics I know never encountered anything similar.


RE:  Booster:


Do these little diameter boosters have a typical spongy feel to them?  Clearly there were issues in my booster with the sticking check vent valve which didn't let pedal release.  But has been working - but then I've never experienced a properly working on - my could be FUBAR the whole time and with no experience with these, I wouldn't know for sure.  My opinion is that it should have more sponge and I do suspect it.


ANOTHER THOUGHT:  The vacuum hose check valve - booster does retain vacuum for 10-15 sec after engine off.  Don't know if that is good nough test?

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After 15 seconds with the engine off, a drop of 25mm Hg is allowable.


After 15 seconds with the brake applied and the engine off, a drop of 25mm Hg is allowable.



Easier to just replace the booster.

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Try pb blast instead of wd40.... wd40 can make rubber swell...

I use to work on motor cycles and I'd have people come in with there chain binding up and acting wierd.... first question I ask is did you happen to clean the chain with wd40?.. they usually say how did you know.. all the rubber orings were swollen....

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Mike, Yeah, screw it, I'm going to replace it.


  • Should I not try to screw around and find a cheap used one?
  • If I get a rebuilt:
    • Any brands to avoid?
    • There are 4 boosters listed for 620 - will they all interchange?  (I think 3 of them are for the L20B models)
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Just a thought...and you're probably on the right track replacing parts....

What if you disconnected the vacuum hose and try the jab test again? Note, the brakes obviously won't respond the same, and it'll require more force. This would eliminate all things related to the booster. 

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I can sometimes get it to flutter when not moving (!)


You can hear the pressure venting in and out under he dash when it flutters.  I wouldn't be a bit surprised if there is a leaky check valve or diaphragm crack that leaks with a very small pedal load - when the load is increased, the leak seals.  FOR EXAMPLE:  When I push the pedal down, it does not develop the "spongy force" - only when the pedal is about to the "solid point" where the shoes touch the drums do I experience any sponge.  So I'm thinking when that sponge pressure is supposed to be increasing as the pedal strokes in, it is not.  Why?  Perhaps a leak somewhere.


I think I can get one at AZ for $85-ish LifeTime warranty.

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  • 2 months later...

"Flutter" was probably caused by crack in bellows - in one position it seals, actuates, but the actuation opens the crack, bleeds down, seals, cycle repeats.


Replaced Booster and all is well - system behaves now as expected.  Only observation is that the small diameter of the booster results in a "smaller moosh" between the pedal and the hydraulics.

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