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| [GMCnet] GMC Fun: Always a New Lesson [message #309954] |
Sat, 05 November 2016 12:44  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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Senior Member |
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Yesterday I undertook the simple, 30 minute job of replacing the GMC's
combination valve. As some of you know, from my 2007 album
http://www.gmcmhphotos.com/photos/g3510-deciphering-
the-combination-valve.html, I disabled the metering valve function of the
combination valve. I've continued to use that old iron-bodid gadget with
my 6-wheel disc brakes satisfactorily until the last few months. During
brake tinkering, I've managed to turn on the "failed system" warning light
a couple of times. The first time it was a little difficult to reset. The
latest time I've had no luck resetting it. So I decided to get a modern
brass disc/disc valve to replace my antique.
Removing the old valve proved a bit of a challenge because I the line to
the rear brakes requires an adapter fitting between the valve and the rear
tube. That fitting hadn't been removed since the valve was built. I had
to put the valve in a vice and really "get on it" to remove it. The flare
nut wrench I was using even slipped once. A lot of trouble just to get a
1/4" female flare to 1/4" male pipe thread adapter!
Planning to quickly switch all of the lines over, avoiding emptying the
master clyinder reservoirs and the consequent bleeding task, my plan was
slightly disrupted by that fitting problem. But I went ahead and connected
the 3 lines for the front brakes, before the brake fluid quit running; and,
I connected the line from the MC for the rear brakes. But that darned rear
tube WOULD NOT line up -- I simply could NOT get the thread started. So, I
removed the adapter, thinking to put it on the tube loosely, then install
the MPT into the valve. WHAAAT! The female flare end of the adapter was
OVAL!!! Remember that slipped wrench I mentioned? Yep, it essentially
destroyed the adapter. :-(
No big deal, aside from all that brake fluid still dribbling out. I'll
just run to NAPA, 1-1/2 miles away and get I replacement. And so I did.
Guess what -- that MPT would not fit the valve! It was "loose as a
goose". Hmmm--calipers, please. Yep, the old fitting's tread OD is 0.025"
greater than the new 1/4" MPT! So, back to NAPA. Even the "old heads" at
that well stocked, usually reliable store were baffled and had to refund my
$2.47.
Off to O'Reilly's. "Oh, that's just a standard 1/4" MPT." Right. 'Cept
it's not, they finally had to admit. On to Advance Auto, where I was
greeted by an old acquaintance who said "Of course we've got it -- what do
you want to bet?" "$2.47". In the back at the fittings bin, he was just
as shocked as everyone else. "Darn, I wish I'd bet you big bucks", I
said. Then I looked down at the Metric section of the cabinet --
Hmmmm...and pulled the drawer that said "1/4 MPT to 1/4 F Inv Flare".
Would you believe -- that was it??! It's apparently a 14mm x "close as
spit" to MPT. On closer examination, it's not tapered and there's a flat
seat at the end of the threads which apparently mates with another flat
surface inside the valve. It cost me $5.67, but at least I didn't lose a
big bet.
It's now installed, not leaking, and the brakes are bled. Putting the
light out wound up costing me a total of about $50, and 3 hours work, plus
a big brake fluid puddle on the floor. But at least that feature prodded
me to do what I've needed to do for years: degrease and pressure wash the
floor. I'm about to go buy epoxy paint. Tomorrow, when the floor's dry,
I'll finally get around to finishing the floor under the service rack.
And I learned another GMC lesson. :-)
Ken H.
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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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