66 396 Brake Problem - Impala Tech
Brakes & Suspension Conversion Questions & more

 
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post #1 of 5 (permalink) Old 06-06-2015, 06:43 PM Thread Starter
 
 
Join Date: May 2015
Location: sykesville md
Posts: 11
 
66 396 Brake Problem

hello,
I installed a new 9" brake booster today I bought from Hubbards,
i believe the original was not holding vacuum,
I bolted to the existing single master cylinder as it looked new.
problem is , now i have a stiff pedal, like they are manual brakes,
the car stops but you have to stomp hard on it,
i checked the vacuum line from the manifold and it has suction,
i checked the drums and brakes and they look good,
could it be the brake booster pin? are there different lengths,
thanks
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post #2 of 5 (permalink) Old 06-06-2015, 08:30 PM
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Posts: 12,458
 
Are we talking a dual nine inch or a single diaphragm?
If it is a single diaphragm, then that is what I suspect would happen. You just reduced the amount of power assist by about half so I would expect the force required to rise by at least that much.

Big Dave
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post #3 of 5 (permalink) Old 06-07-2015, 08:13 AM Thread Starter
 
 
Join Date: May 2015
Location: sykesville md
Posts: 11
 
thanks Big Dave,
not sure , about the diaphragm, i read the Hubbard's booster literature and it doesn't say, i'll have to give them a call.
they include a long/short pins in the kit,
i installed the long based on the other booster,
maybe the short pin is correct?
still puzzled
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post #4 of 5 (permalink) Old 06-07-2015, 10:38 AM
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The long or short pin depends upon what the back of the master cylinder looks like. If the piston is flush with a dimple it takes the short pin, if the piston is recessed with a deep drilled pocket it takes the long pin. Here is what the two look like side by side:



Here is the single diaphragm eleven inch stock booster,



compared to a single diaphragm nine inch booster,



If you had a dual diaphragm nine or eight inch booster it would look like this:



In side view here is the single diaphragm booster off of a Jeep (different master cylinder but booster is the same as GM used);



and the dual diaphragm booster:



which in this case is off of a 1977-82 Corvette. As you can see the dual is a lot deeper and generally has a stepped down nose that reduces the volume of vacuum acting on the diaphragms.

Big Dave
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post #5 of 5 (permalink) Old 06-07-2015, 11:03 AM Thread Starter
 
 
Join Date: May 2015
Location: sykesville md
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thank you for the reply pics Big Dave,
it's a single diaphragm
I'll double check the master cylinder now.
thanks again
Don
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