I bought a brake controller that uses a sensor in the brake line. Today, I
took the brake line apart at the master cylinder, installed the "T" and
sensor. Had my better half pump the brakes while I bled. No big deal, I
Well, as soon as I started the truck, the brake warning light came on. So I
had the better half pump the brakes while I bled a whole can through the
front wheels. No joy.
I called "the Kid". He said to get a vacuum device from bumper bumper for
only $20. Well, it was $50. But I got it and bled a can through each front
wheel. No sign of bubbles at all. Note: the sensor is on the front set on
the tandem brakes
Still no joy. I got a brake warning light. I just got pissed and called it a
day. What do I do tomorrow?
Take the whole thing off?
Maybe post on the automotive newsgroup, and include you make & model of
Writing built in test code for just about any mechanical device is a
bitch and a half -- you've got this computer that is deaf and blind,
that's trying to deduce the health of some mechanical hardware without
being able to sniff or look for leaks, etc. False alarms are common,
even when the assembly _isn't_ broken.
So I'm not surprised that you confused the poor thing.
Either the amount that the pressure sensor moves makes the system look
leaky, or there's still a trapped bubble, or your brake controller's BIT
function remembers when it's _been_ broken and you need to go reset the
Did you bleed the line to the sensor? You may have unbalanced the two
Yea, step 1 above.
What are you talking about unbalancing the circuits???
I just talked with "the kid".I 'll bleed the rear brakes tommorrow. if that
don't work, I'll remove the sensor.
This is why there are few aftermarket trailer brake controllers that tap
into the trucks brake system in any way. It's a big product liability
insurance issue modifying the factory brake system, and it's also a big
DIY installation issue with people not having the correct manuals to
bleed the brake system properly.
You finally said it's a '93 Ford 350, I'm not familiar with them, but
it's recent enough that it may have an anti-lock brake system. If it
does it may require a special bleeding procedure requiring a higher end
scan tool, and I believe it's pre-OBDII, so it's not very standardized.
As someone noted, you need to bleed and dead end line to the pressure
sensor, or you'll have an air bubble stuck there.
I believe Autozone has some manual pages available online for common
procedures like this. You can also buy a subscription to Alldata DIY to
get access to the manuals. You can buy the factory manuals from HELM.
You could try one of the cheapo manuals from an auto parts store, but
you take your chances with them vs. the factory manuals.
I'm a Mopar man, never owned a Ford so flying blind here.
There is usually a valve in the brake system that compares the front &
rear pressures & is happy if they are equal. If one has much lower
pressure than the other the valve goes off-center & trips the light.
On Dodges anyway, if you do work on say the rear brake lines, after you
are all done you must then bleed some fluid out of the front lines to
recenter the valve & turn off the lite. A Google search will explain it
better than I can.
If this 93 has ANY type of ABS you can forget about using a tapped line
for the brake controller. The sensor shows up as a leak to the computer.
Even if it doesn't have ABS it is likely that the sensor has a weaker
spring than the switch in the line which senses brake failure and trips
These are why nobody uses them any longer. They cause a LOT of problems
in modern braking systems.
Plus the better electronic models do as good if not a better job, and
they are easier to install. Just make sure they are level.
Is there a connector for a pressure differential switch on the front
end of the master cylinder?
It's shown on the Aerostar in my 1991 service manual.
there's an electrical connector under the middle of the plastic reservoir.
The reservoir feeds both the front part of the master cylinder and the rear
part. I didn't try very hard, but the connector doesn't want to come apart.
It is a pressure sensor, not a pressure switch. It has essentially no
displacement and will not show as a leak of any kind. The factory
integrated trailer brake controllers use such sensors.
Most likely he has an air bubble trapped in the sensor barrel, as well
as potentially having an ABS bleeding issue.
There's a "low-tech" way to reset those valves.
First, there's no indication whether it's the front or rear that's caused
the imbalance. That means you'll have to work on both circuits before
you get it.
Put a bleeder hose onto (say) the front, and have a helper go through the
exercise of filling the hose until there's some fluid in your reservoir
so it can't back bubbles into the system. Y'know... push pedal, crack
the valve, close the valve, release the pedal... and so on.
Once the hose and catch bottle both have fluid in them, have your helper
push on the pedal fairly hard, then crack the valve open more than just
for bleeding. Have the helper make sure they don't lift the pedal until
you close the valve again.
Now you've deliberately off-set the balance valve to the front. (front
pressure low condition).
Now go to the rear. Repeat the exercise, but SLOWLY AND GENTLY pressing
the pedal while slightly cracking the valve AND WATCHING the light. At
some really twitchy point in the pedal travel, the light will go out.
Helper hollers "OUT", and you simultaneously close the bleed.
It's worked for me a dozen or so times. Tricky, yes. You might have to
do it three or four times before you get it right.
Clue here -- when watching the light, if it blinks off then on again,
you're "past center". Then it's the other end of the vehicle you need to
repeat the operation on.
If it's on the reservoir, that's a float switch to light up an idiot
light for low fluid level. Some replacement reservoirs have the float
inside and the physical space for the plug, but no contacts, go
figure. Usually you can't pull the plugs directly off on Fords,
there'll be a lug on plug or socket and a mating loop that has to be
lifted oh-so-carefully to clear the lug before it can be pulled off.
Sometimes there will be two on opposite sides. I use a jeweler's
screwdriver to do the lifting, as they get older, they get crispy and
the loop bits break off all too easily. They don't sell that stuff,
either, replacements can only be had off wrecks Fortunately they're
easy to unpin and repin.
An important safety note now that you've indicated that your tow vehicle
is a 350:
"A nearly new gooseneck trailer followed me home from the auction. Its a
deck over dual tandem axles, 16" truck tires, beaver tail with flip down
ramps. I could safely haul a 25,000 lb. machine using my truck."
Your safe hauling capacity is substantially less than the 25,000# that
you noted earlier.
The maximum 5th wheel / gooseneck Gross Combined Weight Rating (GCWR)
for any configuration of a 2009 model year F350 is 27,000#. The lowest
rating for a DRW configuration is 10,400# and I presume you have a DRW
since you indicated a dump bed. I also believe a 1993 model will be
somewhat less as they have raised the ratings over the years.
I will use the 2009 F350 ratings and presume that your 36' dual tandem
trailer has a dry weight of 5,000# for these calculations, but the
trailer may well weigh more and the trucks GCRW may well be less. I will
also use the lowest possible curb weight for any F350 DRW configuration
of max GCRW capacity at 7,024#. For the dump bed I will assume 1,000#
over a standard pickup bed.
GCRW: 27,000# GCRW: 10,400#
Curb: -7,024# Curb: -7,024#
Dump: -1,000# Dump: -1,000#
Trlr: -5,000# Trlr: -5,000#
Cargo: 13,976# Cargo: -2,624#
As you can see, no possible configuration gets you anywhere near 25,000#
of cargo capacity without vastly exceeding GCRW and the worst case truck
configuration with wimpy engine is over GCRW with the trailer empty.
Please run this calculation with the actual GCRW for your truck
configuration and the actual trailer weight so you know what your safe
cargo capacity really is. This is particularly important since you might
be considered commercial, both from your farm, as well as Federal Motor
Carrier Safety regulations due to weight.
If you want to tow this trailer at full capacity you will need at least
a class 5 truck (F550).
Karl, hazy recollection of this on earlier Fords.
Should be a block on the left fenderwell area with brake lines, 2 in (F
& R) and two out (ditto).
Little capped boss top center, with a wire sticking out. When the
differential gets upset, the sliding valve in the center of that block
moves to one end or the other, and turns on the light.
IIRC, you have to center that piston, leave a tool in there to hold
the piston centered. Bleed the lines, then then reinstall the sensor.
Light should be off then, good to go.
I have a 1996 F350, but it's not where I can look at it right now.
Besides, it has ABS. Not sure your 1993 does. But that may not make a
difference WRT to above procedure, as I think the ABS they used was a
Thanks, I'll look for this.
I tried the bleeding the back brakes to balance. No joy. So i took it out on
the raod and hit the brakes hard. It stops great, but now I got an ABS
warning light too. There's a reason I quit trying to maintain automobiles
after the late 70s models.
I'm about to just take it in to the shop.
On Wed, 24 Mar 2010 17:32:39 -0500, the infamous "Karl Townsend"
scrawled the following:
When you bled the rear brakes, the sensor piston went forward and hit
the alarm. Then when you bled the fronts, the piston went backwards
and triggered the alarm. You need to center the piston so the alarm is
In the olden days, I'd quickly open and close the bleeder screw to do
that. It was hit or miss. I believe that some sensors come with a
centering hole which you can insert a tool, but I got out of the
industry back in '86, before a lot of anti-brake hardware and
electronics were installed. Consult your local dealer for that info,
or buy the service manuals for your particular vehicle, Karl. You'll
want it anyway.
If we attend continually and promptly to the little that we can do, we
shall ere long be surprised to find how little remains that we cannot do.
-- Samuel Butler