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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.
 
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