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disc brake update- kinda long

From: <lpedersen_at_ucdavis.edu>
Date: Sun, 10 Feb 2002 20:53:18 -0800 (PST)

Anyone who has noticed my previous brake posts may be interested in what
I've learned and how I'm now set up.

For background, I had installed the BC Broncos rear disc brake conversion,
which was complementing my stock Ford front disc brakes and power brake
setup. I had converted to a 1 1/8" master cylinder (Napa 39445) from a mid
eighties f250. At the same time I changed the master cylinder I installed
a rebuilt booster from Napa, since my old one was leaking and causing my
SEFI setup fits. My pedal was soft and touchy for about 2" of travel after
which it became rock hard as though it hit a stop. Actually, I could push
harder and it would stop faster, but only if I used 2 feet. Very strange.

After changing to a Lincoln 4 wheel disc 1 1/8" master cylinder with no
improvement, I got concerned that it was the rear discs. I proved that to
not be the case by blocking the rear brake port on the master cylinder
with a plug I got from Napa. It didn't change the pedal feel at all- still
mushy then rock hard.

Chuck was extremely helpful while I was trying to figure out what was
wrong. He runs the Ford front calipers, the same M/C, and obviously his
rear disc setup. I figured the only thing it could be was the booster, and
he kindly loaned me a spare stock booster (his power brake kit uses a new
booster that he has made, it's different from the OEM Bendix booster that
EB's came with.) Lo and behold, that was the problem. Interestingly, the
rebuilt booster that Napa sold me (new OEM bronco boosters are
nonexistent) was apparently a booster modified by Duffs to emulate the OEM
booster. It has a welded-on bracket for the bracket studs, where the OEM
booster has the studs coming directly out of the booster case. So the
custom bronco stuff is finding its way back into circulation. My advice
for anyone considering a power brake conversion would be to use the BC
Bronco kit, since you then get a new booster- no dealing with ancient
parts rebuilt by people with considerably less interest in your truck's
performance than you have.

While I was on a roll, I scrounged a pair of the big Ford front calipers
from a local yard. They require a special hose to fit the bronco- the
stock caliper has a hose that goes straight in, which interferes with the
upper ball joint. We need a banjo fitting, but it's larger (7/16-24) than
the stock bronco part (3/8-24.) In talking to Chuck I found out that he
carries the hoses for this swap.

I gutted my stock 76/77 proportioning valve while I was at it- disabled
the metering valve in front and the proportioning valve in the back,
leaving the differential valve (turns on the dash light) intact.
Interestingly, this valve has a residual pressure valve built into the
front circuit. I did this because with the big calipers in front I guessed
I wouldn't need a proportioning valve, which was meant for rear drums
anyway. Even if a proportioning valve was needed for some applications I'd
recommend the Wilwood valve to retain control over the brake bias.

The end result is that my brakes are sweet. The $hitty rebuilt booster was
the problem- the pedal is now linear and firm, and there is no tendency
for the rears to lock up first, even in dirt. I'd recommend the BC Broncos
rear disc conversion to anyone considering the swap. If you've got Ford
front calipers, the swap to the big caliper (Todd Z has the cross
reference of vehicles that used this caliper, otherwise BC Broncos stocks
rebuilt ones) is worthwhile, particularly with big tires.

Lars Pedersen
Davis, CA
Received on Sun Feb 10 2002 - 21:53:41 MST


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