2008 Dakota 78,000mi AutoTrans, 4WD It appears to have 2-Wheel ABS Brakes since the Computer Scanner on Live Data shows "SNA" (sensor not applicable) on right front and left rear position. I drove the truck and verified I had accurate Wheel Speeds on the LF and RR wheels,

The master cylinder started bypassing fluid, so the pedal when depressed initially applies the brakes, but with medium brake pedal pressure, the pedal will slowly depress toward the floor.

No brake leakage. No Brake or ABS Warning Light with the OLD bypassing cylinder. Only symptom was pedal moving down.

Obtained new Master Cylinder intended for 2 wheel ABS brakes (Appeared identical to old one) and bench bled it.

Replaced Master Cylinder with NEW (not remmanned cylinder) and filled reservoir to "MAX" level.

Brakes now work perfectly. Solid pedal, and no movement once depressed. Road test indicates problem solved.
Again: There is no air in the system, and the pedal is rock hard and unyielding to leg pressure.

I thought I was home-free.

PROBLEM: NOW. I have a "BRAKE" and "ABS" Lights and the scan tool shows "Low Brake Fluid."

I pulled the electrical plug off of the master and re-seated it "just in case." STILL shows Brake and ABS lights, and scanner shows "Low Brake Fluid" that will NOT reset.

Suspected maybe a bad fluid level switch in the new master (since I know the reservoir is FULL), so I plugged the OLD Master outlet fittings, fastened cylinder in vise and measured resistance of level switch on old cylinder empty. I pumped it dry to make sure, in fact I was measuring "empty". It read 3 ohms

I then filled the OLD master FULL, bench bled it, and re-checked the switch resistance. STILL 3 ohms.

So I am guessing the low level sensor is a Hall Effect or something? It has only 2 terminals, BTW. ANYONE KNOW?

So... Opinions PLEASE-

1) Did I get a BRAND NEW Master Cylinder with a BAD Level Sensor, or... (Is there a way to test the sensor?)
2) Is there something else going on?

Thanks-
Bob

Note:
Last year this truck drove me NUTS when one of the original front wheel calipers started dragging, and got HOT. I changed BOTH calipers and now BOTH dragged. I changed the hoses, and it appeared the pressure was retained in the calipers themselves, so I exchanged the calipers for new ones and the problem went away. Great brakes until NOW and the Master Cylinder problem.

 

Ugh sorry for your issues. Could the ABS sensors on the wheels hubs cause something like this? I myself had had a number of issues with the front brakes of my 2005 SLT 4x4. Sorry I do not have any more information to help your master cylinder issue, but I too have had to put 2 different sets of brake calipers on the front. My OEM caliper started dragging on the passenger side. I replaced them with powerstop calipers and then again after a few months the passenger side started dragging. Sent them back and got replacements and replaced both side brake hoses while I was in there. A few months later same issue, started dragging and would get real hot. Went and got ACDelco calipers and did a full fluid swap. I noticed my master cylinder had a hole the rubber seal of the fluid reservoir cap. I thought maybe my fluid was the problem since it was original and might have had water in it. Since doing all of that its been good, but I always suspected that maybe the master cylinder itself might be the problem. Maybe the proportioning valve or something. Good luck!

 

These vehicles have either 2 or 4 wheel ABS, but this one has only 2 wheel ABS with one sensor on the LF and one on the RR. I checked the sensors by reading the wheel speeds on my scanner in Live Data mode.

The "Brake" and "ABS" illuminate about 5 sec after engine start, and code thrown is C1009 whis is "Low Brake Fluid Level." The fluid level is near max on the reservoir, so I suspected the level sensor in the new master cylinder was bad.

I obtained a second new master cylinder today and installed it with unfortunately the same result, meaning that either I was given TWO bad master cylinders or the vehicle suddenly developed a problem after removing the old master cylinder (we did NOT have the BRAKE and ABS lights on with the old master cylinder).

After having TWO new masters show low fluid level, I decided to plug the outlets on the OLD master, fill it with fluid, and plug the level sensor plug into the old master since it did not show low fluid level. THIS time however, even the OLD sensor shows low fluid, so obviously the problem is in the vehicle itself and not in the master cylinder.

I suspect the ABS light illuminates automatically whenever the Brake quantity is low, so there is just one fault-the low quantity issue the system perceives but is not fact.

The sensor plug on the harness is perfect, neither wire is loose in the plug, and the wires appear to be in perfect condition and disappear into an armored sheath that in turn disappears into the fuse box with no evidence of physical damage,

 

I thought when changing a master cylinder on an ABS vehicle you have to prime the ABS sensor using a scanning tool, is that true? I have heard that the tools the dealers have can run a test where it primes the ABS system so it’s prepped for a new master cylinder. Do you think maybe that needs to be done? No idea, just a guess since the old sensor is now throwing the same code after swapping it out.

 

I have a 2011 and show about 10k ohms across the brake level sensor with the reservoir full.

You sure the float isn't stuck in the down position?

 

You did reset the abs computer I take it?
I just replaced mine an no problems after full bench bleed an bleed of lines an reset abs. But mine does not have the wiring at the reservoir an is 2wd 3.9,an not 4by4 ..
Some 4by4s may need the Computer bleed process to get air out of solinoids but my 2wd did NOT!

 

I installed a new reservoir from auto zone and had same low fluid code C1009. I took the old original OEM reservoir, cleaned it and hooked it back without fluid.. As soon as I turned the reservoir upside down, the light cleared. So, have come across two issues. Bad sensor or wiring is breaking continuity. Unfortunately, the replacements are possibly an issue, too. Mention on various forums say installing a 50w-1000ohm resistor on the ground wire coming into the reservoir may clear the problem...but it won't tell you if the fluid level is low if that happens in the future. Since I'm selling this truck once I fix it, I'm going to try the fix while I order a OEM pump/reservoir from dealer(if they are available, which I think they stopped making/selling them for a 2006 Ram 1500)
Crossing my fingers, it's been a long, frustrating week.