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Brake Booster Boost Output is Weak: What to expect? Normal?

Cardinal Grammeter

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My experience with PB is that there is a "spongy area" where you are getting boost, but when you push harder, you get to a point where the boost is maxed out and the booster "goes solid" and you are now directly pushing on the master cylinder piston.


I get to the "solid" event with pedal at 2/3rds stroke.  When it goes solid, brakes are hard and stop just fine - and without any kind of excessive pedal effort.


In the "boost area" of the pedal stroke, the amount of boost is just noticeable when I get near the "solid" stroke point.  (If I had to guess the max boosted level of braking would be 1 out of 10 where 10 was brakes locked up.  The amount of boost doesn't amount to much at all.)


I would expect more "boost" from PBs but the diameter of the booster is so small, I can't see how any kind of boost could be generated. 


However, I'm doing all this testing at slow, neighborhood speeds.  If I was going 50 mph and jammed on the brakes, there would be a lot of engine braking, high vacuum and more boost. 


There may also be issues of low vacuum in the intake: 


I set the timing according to FSM and it backed the vacuum level out of the "green region " on the vacuum gauge.  I don't know what the total timing is, but I never set timing at idle - its always at full advance.  Engines like in the low 30's to 40* total, where does the L18 like to be?  Best test will be to simply road test, advance idle to just where it does not ping.  That has always been the way to do it in the 60's.  In fact, that is the recommended procedure in the Studebaker Shop Manual for all their cars up to 1964.


When I work the brakes in the solid zone, its like what I would expect from manual brakes.


If there was air in the system, the solid zone would not be solid.


This truck has had lots of corrosion damage and I'm wondering if the PB booster could have rusted/stuck internals?  I don't think there is a vacuum leak because when idling and using the brakes, there is no change in engine speed.




EDIT:  New Theory:  Brakes not adjusted tight enough:  When the brakes "go solid," I've used up all the PB booster travel?


The rears are totally backed out and can barely get the drums on - new brakes that tight???  But the fronts are not that tight and do not have that problem.  I'm thinking I could tighten up the fronts.


NOTE:  On my very first drive on roads, a rear brake was locking up the instant brakes applied.  I figured that brake just needed to be "run in" and after 5-10 minutes of driving around, that symptom disappeared. 


EDIT - 2:  Not the front brake adjustments.  Could only tighten 5 clicks each side.


Can't be air in system - the pedal travels about 1/2 way while nothing happens, then it goes solid with no trace of sponginess.


Maybe there is no problem and it is typical for the brake pedal to go 1/2 its travel to go solid and work.

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Back to original question after all brake issues solved:  Is the Power Brake unit boost weak or wimpy?


Booster passes all "pedal tests" but simply does not feel like any PB I have ever driven:  all PB cars have a "mush" to the pedal that never "goes solid."


My PB has this mush and is definitely an assist for casual driving.


However, if I want to get into the brakes just a little, I "use up the mush travel" and feel the pedal "go solid" and I'm back to like having manual brakes.  Of course, the PB unit contributed some force which helps - but only a little.  Guessing, the boost would have to be maybe 4x what it is now, to keep me from getting to the "goes solid" point.


My concern is that with such a tiny PB diaphragm area, there simply cannot be a lot of boost generated by the unit.




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If you want to compare boosted and non boosted brakes just pull the hose off and seal the intake side. Be sure to try this far from any danger of hitting anything. You'll be amazed how hard it is to stop without the booster working. Use extreme caution it's like not having any brakes at all.


The brake booster has a one way valve in line that stores vacuum in the booster even if there is none in the intake from a stalled engine. You would get at least one good step on the brakes and get stopped. If this ever happens, a stalled engine while moving, never pump the brakes and waste the vacuum.


The system works, so if yours does not, it's not the fault of the design but a possible problem with your booster or the master of the brakes themselves.

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Wow are you right!


Yesterday I went on a long drive and the temp gauge started getting very high - was between the last 2 marks - so I decided to shut engine off going down hills.




Booster is putting out LOTS OF BOOST!


I think that "going solid" is a behavior of drum brakes - its when the shoes actually seat against the drums.


Anyhow, this one is a Case Closed.


Thanks Mike

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Try this. Pump brakes to release any stored vacuum. Depress brake moderately and hold. Start engine.


What should happen is the pedal will drop towards the floor slightly. This is the intake vacuum pulling on the booster diaphragm and is normal.



If you place a vacuum gauge between the inline one way valve and the booster and start the engine you should get around 19-20 inches of mercury. When shut of,f the vacuum should not drop more than one in. of mercury in 15 seconds.


Now start engine and apply and hold brake. Shut off engine when 19-20 in. of mercury is reached. The leakage should not exceed one inch of mercury in 15 seconds. 

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I suspect I'd fail that leak down test. 


Remember the vent valve was sticking: brakes would not release - until I "beat on them with my foot" until I heard a loud click, and then they did.  Then I slopped Teflon Garage Door Lubricant down the rear opening of the booster (I took it off.)   It seemed to actuate fine on the bench - I ran engine vacuum hose to it and pushed the pushrod in by hand.

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  • 1 year later...

CONCLUSION:  Booster was starting to fail/maulfunction.


Replaced booster due to "fluttering" and a slowly escalating vacuum leak  The "going solid" was not present in the reman booster. 


Further the output pushrod of the old booster could be pushed sideways to about a 45* angle - this was NOT possible with the reman one.


The reman booster behaves like a typical booster and does not have any idiosyncrasies albeit that the small diameter results in a "smaller" "smoosh" feeling when applying pedal. 


Oh, and there is more stopping power too - nothing dramatic but definitely noticeable.

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