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Discussion Starter #1 (Edited)
Recently, I experienced a severe brake failure at speed,
which almost resulted in a rear ender on an expressway.
I stomped on the pedal and instead of normal brake reaction, the ABS and Brake light came on,
and my truck had barely 25% braking function left! After crawling to a side street
to stop and check out the master cylinder, it was completely empty!
Both sides of the reservoir had no fluid, and this is supposed to be a dual braking system on these!
,
Managed to crawl slowly to a nearby garage. They found that the front to rear brake line
(the one controlled by the ABS), had opened up due to rust, and lost the
brake fluid, rendering the back wheel braking useless.

Ok, but the front brakes which should have been a complete separate
system barely had any fluid at all! Although the pedal wasn't touching
the floor (yet), it barely had just enough left to stop the vehicle at slow speeds.

I had the front to back brake line, over the axle lines and flex hose
replaced along with two new wheel cylinders and shoes, so my brakes
are back to normal..but...

I lost the fluid in the front braking system and I am still annoyed that
this can happen on the Dakota.

After checking out the braking system more thoroughly, here's what
I found on mine..

The Master Cylinder has two separate fluid compartments in the
reservoir and a dual piston with separate lines coming off it.

These go to a combining/proportioning valve that has a dual piston
in it and brake light switch coming off it. The FRONT of the MC goes
to the REAR of the combo valve and vice-versa.

The output of the combo valve:
FRONT goes to the HCU (hydraulic control unit),
which has it's two solenoids (dump and isolate solenoids, I think),
these are controlled by the ABS controller.
The o/p of the HCU goes down to a distribution block on
the frame and a line goes from it to the rear wheels.

REAR goes to a distribution block on the frame and two lines come off
it going to both front wheels (non-abs).

This Combo valve is supposed to isolate either the front or the rear
braking system depending on which system loses pressure (brake failure).
In my case, it appeared to have malfunctioned (sticking?) and because
of that there was excessive fluid loss in the front system because of it.
 

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Discussion Starter #2
Brake pedal goes down initially after sitting overnight

I've had this problem for a few years and can't seem to put my foot on
it. :D

Sometimes after the truck sits overnight and I first start it off in Reverse
and hit the brake, the pedal seems to go down farther than it's supposed
to..almost like there is no power booster working.

Hit the brake a couple times and everything is fine..full pedal and hard.

Not sure where to start on this one..anyone have an ideal?

Master cylinder?

Proportional valve?

Vacumn power brake booster?

I've had this problem for a few years now and it doesn't always do it..
it can go for many days or even a month and the symptom doesn't
appear..then if shut off overnight and started up, if I don't hit the brake
pedal a couple of times, the pedal goes down further than it should.
Both front calipers have been replaced in the last 2-3 years.
Last year the front to back line, wheel cylinders and brake shoes were
replaced.

Still original are:
Master cylinder
proportioning valve
vacumn booster

Not sure where to start on this one..anyone have an ideal?
Master cylinder?

Proportional valve?

Vacumn power brake booster?
 

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probably neither; a bad booster will give you a rock hard pedal and possibly a hissing noise if it has a vac leak;
other than air in the lines (doubtful if it isnt always low) low pedal would then most likely be out of adjustment rear drum brakes; the star wheel adjusters Don't always work like they should; I doubt it would be a master and really doubt a bad prop valve; possibly an occasionally sticky solenoid (dump valve) in the rear ABS? also a shot in the dark as well; check, clean and adjust the back brakes and see what happens... "in the last year" means nothing; I have 14K on my truck in 5 months' of use.... and mostly open road at that; so my brake usage would be different from yours.... driving patterns etc. that brake pedal is probably how it is ,more often than you think...
 

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and the rear brake line is NOT "controlled by" the ABS; it is controlled by your foot the master cylinder and the wheel cylinders; the ABS does NOTHING unless called upon; my 94 did a similar thing, and it wound up being that I blew the rt rear wheel cylinder... fortunately I was able to find and fix it right away before my shoes became saturated with fluid; a quick spray off with Brake Cleaner was all they needed after replacing the cylinders; if shoes get saturated they gotta be replaced regardless of how much meat they have left on them...
 

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Discussion Starter #5 (Edited)
I doubt it would be a master and really doubt a bad prop valve; possibly an occasionally sticky solenoid (dump valve) in the rear ABS? yours.... driving patterns etc. that brake pedal is probably how it is ,more often than you think...
Got my answer this morning.



First of all, yesterdays weird brake symptom;
started the engine and put truck in reverse and went to brake..and brake
pedal seemed to be awful low and soft.. but then the pedal seemed to be
ok. Drove around a bit in town for groceries and a new air filter for it in
preparation for todays 60 mile trip to the campground.

But something inside told me to double check the brakes again before driving
off to see if the same symptom occurred.

Checked Master cylinder, it was on the add mark but not excessively low
to indicate a problem. Topped up to full mark with DOT3 fluid.

Started truck; pressed on the brake pedal..seemed ok.
Pressed again..hard this time..seemed a bit soft for some reason and the brake
pedal seem to travelled down a bit more, with engine running.

Pressed again..about 3 or 4 times hard...on about the 4th time the pedal went
down almost to the floor (but not quite)..and I smelled brake fluid.

Checked underneath and the line to the left front wheel had split somewhere
and fluid was pouring out.

Called CAA to load truck onto flatbed and tow it into the garage where they
will examine the braking system, replace both sets of steel lines on the front
and the hoses.

Note: front to rear line split last year, and it was replaced along with the
axle lines and rubber brake line at the back. Both wheel cylinders were
changed, new shoes, brakes adjusted (about 3,000 km on truck since then
so I had no reason to suspect the back brakes, yesterday, so I was trying
to determine if the master cylinder was causing the symptom, not realizing
that the brake line may have started to leak even yesterday..rusted out.
Even a tiny rust pin hole can cause a soft brake pedal.

Both front calipers have been replaced (due to seizures) in the last 3 years
or about 15,000 km, but not the front lines or rubber brake hoses.

Master cylinder has a minor leak for several years now, but not severe enough of a leak, more like seepage down the vacumn booster, that only occassionaly fluid had to be added.


This seepage has bothered me for some time now, and so has the low initial brake
pedal syndrome. Since I'm going to sell the truck and need to get it safetied,
I'm going to mention that to the mechanic to change it out while replacing the front lines at the same time, since the entire brake system would need to be bled.

This low pedal syndrome at startup/first depressing the brake has been there for
several years now. First noticed it about 3 or 4 years after I bought the truck in 1998.
So it's been like that for about ten years.

About 10 years ago, brake fluid seepage running down the vacumn booster and eating the black paint off so it started to rust there after that. But it was not consistent seepage, so I didn't do anything about it up to now. I've sprayed someoil on the rust spot of the vacumn booster below the master cylinder, but the solution probably should have been to change out the Master cylinder a long time ago.
 

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Discussion Starter #6 (Edited)
probably neither; a bad booster will give you a rock hard pedal and possibly a hissing noise if it has a vac leak;
No hissing and the brake booster vacumn diaphram works normally.

other than air in the lines (doubtful if it isnt always low) low pedal would then most likely be out of adjustment rear drum brakes; the star wheel adjusters
If it was air in the lines, (all the lines were bled last year when the
front to rear line/wheel cylinders and shoes were changed.) the pedal
would be spongy all the time and it didn't seem to be.

Pedal was rock solid for a while..sat around most of the winter , now when
I go to use it, the pedal seems when starting up and depressing it the first
time. It goes down farther than it should but then the pedal seems to firm
up again.

Don't always work like they should; I doubt it would be a master and really doubt a bad prop valve;
Last year when the rear line split, I lost ALL the fluid in both compartments
of the master cylinder. I limped in to a side street off the expressway ramp
where the line blew. Crawling in 1st gear and depressing the pedal a few
times to get to the side street. When I opened the hood, the master was
empty of fluid. This is supposed to be a DUAL master so that if the front
line is broken, you still should have some rear braking left to stop you
or vice versa. The combination (proportioning) valve is designed to
ensure full pressure in one circuit should the other circuit fail. The brake
differential pressure switch compares the front/rear pressures and if
there is a pressure failure (open line) it brings on the Brake warning light
on the dash. Because of rear line failure, the ABS light also came on
last year...but not this time.

(The master goes to this proportioning valve that has
the brake pressure switch on it. )

I don't know if it was the master that was defective or the proportioning
valve stuck..but I lost ALL the fluid in the master, and that shouldn't
have happened. The rear line was replaced, the brakes bled all around and
the pedal was firm after that. I checked the master reservoir for a few weeks
on a daily basis, but the fluid level stayed up. Why the all the fluid got
dumped in both compartments is a still a mystery to me.

possibly an occasionally sticky solenoid (dump valve) in the rear ABS?
If it was a sticky dump solenoid ..maybe? Unfortunately with the RWAL
(Kelsey Hayes ABS) there may be a code when the ABS light comes on,
but you can't read it directly from the OBDII connector.

If you have the right scan tool for the CAB, there are codes stored in
the ABS controller.
Open isolation valve circuit
Open dump valve circuit
grounded/closed RWAL valve switch
erratic speed sensor
shorted dump valve
shorted isolation valve

but none of these have appeared normally with an ABS light, in my case.

When the rear brake line failed, I did get an ABS and a Brake indicator
though, but at that time didn't have the tools to determine if the ABS
controller had an issue..neither did the garage that replaced the brake line.
 

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Discussion Starter #7 (Edited)
and the rear brake line is NOT "controlled by" the ABS; it is controlled by your foot the master cylinder and the wheel cylinders; the ABS does NOTHING unless called upon;
Yes, I understand a bit about the RWAL on the Dak and that system (Kelsey-
Haynes) is (or was) used on GM/Ford trucks as well.

The ABS kicks in only when it detects a wheel lockup in the rear wheels, otherwise it shouldn't kick in if you are braking normally and the wheels still
keep turning a bit. In otherwords, the VSS sensor in the rear diff is still sending
out some frequency of pulses to the CAB (ABS Controller). The Isolation
valve is there to isolate the front braking system from the rear at that moment
and the dump valve solenoid is there to "dump" the pressure momentarily
in a pulse mode to prevent the rear wheels from locking up.

If the VSS sensor didn't work correctly (where the ABS controller gets it's
wheel rotational signal from) the brakes are designed to work as normal
brakes..no ABS action and you will get an ABS indicator lighting up as well
as the speedometer not working correctly on the Daks.

I have seen this on a 1999 Dakota and the brakes were rock hard. When I
advised the owner that the VSS supplies pulses to the CAB and speedo,
he changed out the VSS and it fixed his problem..no more ABS light and
speedo was working again.

I only have seen an ABS indicator come on last year when the front to rear brake line blew out. Since the RWAL ABS has nothing to do with the front
calipers on the 2wD Daks, the ABS indicator won't come on even if you
blow out a front line..only the Brake indicator light..which is turned on by
the brake pressure switch on the combination-portioning valve.

In my case the initial soft pedal couldn't have been caused by the ABS
since it could happen backing up as well until I hit the brake a couple of
times to build up the pressure...after that it was fine..unless it sat overnight
and then it doesn't happen all the time.

What really annoys me is the appearance of total loss of fluid and almost
no braking when the rear line blew last year. You would think that with
a dual braking system, this shouldn't happen but BOTH the front and rear
master cylinder lines go to the combination valve and perhaps
some kind of pressure "malfunction" there, could redirect all the fluid from
the front part of the MC into the rear?
 

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Discussion Starter #8

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Discussion Starter #9
May 14. Replacing the master cylinder on my 98 Dak. The low brake pedal syndrome after
it sitting overnight and the seepage at the back of the MC down the power booster for the
last 5 years basically indicates to replace the MC along with the front brake lines and
hoses to the calipers.

After all it's a 14 year old truck now and parts need to be replaced more frequently.
Brakes being a safety issue is something that can't be ignored. I'm glad that
I persisted in pressing on the brake pedal hard a few times in my driveway causing
the line to blow out..better than having that happen on the highway at speed in a
panic stop. At least I'll be safe with any occupants in my truck, not to mention the people in the other vehicles.
 

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Discussion Starter #10 (Edited)
Well I got my truck back today. All the steel lines and rubber hoses on the front wheels. replaced with a new master cylinder. Almost a $1000 with parts, labour and our 13% tax up here.

When I discussed losing my all my fluid last year when the rear line split, (with the
mechanic who replaced them) he kinda chuckled and mentioned that even though the braking system is supposed to be
separate for front disc and rear drum, it's not really a truly isolated braking system

like in the "good ole days". If a steel brake line splits, and you keep pressing the pedal, you will (eventually) dump all the fluid in the reservoir!

The Master cylinder design used in my Dakota ( in case of a split line), is meant for basically just one brake application to slow you down, or stop you, so you can pull over to the side of the road and call for a tow truck.

This is the way it is apparently.
In the old days, I had a 78 GM Pontiac and it had a true dual system master cylinder. I remember when the rear lines split on me, I was still able to crawl home because the front compartment of the MC was still full of fluid and the front brakes were still working.

So beware out there Dakota owners..make sure you have your brakes and lines
inspected regularly.
 

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Discussion Starter #11
Well this morning I got another surprise with my 98 and it's crappy oem braking system!

In the last 3 years (209/2010) I have replaced both front crappy oem calipers
and all the ball joints as well as all the brake lines.

The garage that replaced both calipers at the same time, must have used crappy discount parts, because the same caliper (LHS) seized again on the highway in 12 months and
8,000km (5K miles). I had the LHS side caliper replaced again and I couldn't get
any satisfaction or credit from the garage that installed them in 2009. I would never
use them again!

Now this morning when I went in to get a safety check performed (selling this money
pit) after spending nearly $3000 in parts and labour in 2010/2011 and recently new
front brake lines/hoses and master cylinder (another $1000 with tax), the
safety check mechanic tells me that the rhs wheel bearing (which was supposedly replaced last year...is loose. Now this is a mystery because it has been less than 8,000km on the truck since the wheel bearing hub was replaced, and it is the SAME SIDE as the one the garage identified was loose about a year ago..unless they changed the wrong side when I came a couple days later to replace it?

The pads (semi-metallic) are wearing down more on one front wheel than the other and I noticed my gas mileage has dropped off 20%, so that caliper which may have been the one that was replaced in the last year or two is DRAGGING AGAIN.:confused:

These Dakotas seem to just eat brakes!:(
 

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Although 2/3 videos are no long available due to copyright, this thread really helped me better understand the braking system. Thanks a lot!
 
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