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| [GMCnet] Brake Booster Peculiarities [message #178672] |
Wed, 01 August 2012 17:31  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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Senior Member |
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Admittedly, I've never given a great deal of though nor study, and no
tear-down/analysis to vacuum brake boosters. But I did think I understood
how they work: Engine (or other aux. source) vacuum is stored "behind" two
diaphragms until foot pressure opens a valve to atmosphere. Then,
atmospheric pressure acting against the unsupported diaphragms provides the
force to actuate the master cylinder pistons.
That conception said to me that the booster and the master cylinder are two
cooperating, but stand-alone, systems. Today's events have me wondering:
A friend is in the process of installing a vacuum reservoir and a pump. As
a retired Delta Airlines pneumatics technician, he's knowledgeable and
cautious, so he tested each component before assembling the system. When
he got to the booster, he connected a vacuum pump and a gauge, intending to
draw the vacuum down and monitor the booster's leak-down rate. He was
surprised to find that he could only pull 5" in-Hg of vacuum!
After scratching his head a while, he called to ruin my day too. Baffled,
I volunteered to set up a similar test on the workbench using the booster I
removed last year. I thought that booster was OK, but I bought a
sensitized one from Leigh Harrison -- which doesn't feel very "sensitized"
to me.
My setup is vacuum pump, tee to gauge, then to a check valve at the
booster. All connections are good -- it will hold a vacuum up to the check
valve for a long time.
Now the surprise: My re-purposed icemaker compressor will easily pull an
idicated 29" in-Hg vacuum, and pretty quickly. But connected to the "good"
booster, it would only draw 2" in-Hg, and even I could hear air flowing
into the MC side of the booster. Hmmmm... I had a large soft plastic pipe
cap which just fit the recess on the MC side of the booster, so I plugged
that hole. The sound went away, and the vacuum immediately rose to 29
in-Hg.
On the pedal side of the booster, the pedal push rod passes through a ~1"
OD black plastic "sleeve". When I depress the pedal push rod, the rod, AND
the sleeve, move into the booster; the vacuum drops on the MC side, and, I
presume, the MC push rod moves toward the MC.
Questions for those who really do understand the booster:
(1) Should the MC be sealed to the booster in order to seal the vacuum, as
my plastic pipe plug does? Would that not expose the MC's hydraulic seals
to unwanted stress?
or
(2) Should the booster have an internal seal for that vacuum?
(3) Is the booster I tested (and my friend's mounted one) bad, or just
missing something?
(4) Was the boosted action I saw due to only one of the diaphragms, with
the other leaking?
(4) Has anyone tried DIY repair?
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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