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Old 03-20-2017, 07:46 PM
BrettS BrettS is offline
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Join Date: Oct 2006
Location: Fairfield, PA
Posts: 91
The problem returned when I started the engine.

I declared victory prematurely. I've been trying for several days to figure this one out, but I'm stumped.

As I mentioned in my problem-solved post, I had a firm pedal after bleeding,. But after I posted that, I started the car. As soon as I started it, the pedal went (nearly) to the floor. With the car running I can pump up the pressure, but the pedal sinks (rather quickly) as soon as I stop pumping.

It is very consistent in how I can pump up the brakes with the car off, and the pedal remains firm. But as soon as I start the engine, it sinks.

I've swapped out the master cylinder (out of desperation) with a used one, but it is known to be good. Nothing changed.

I plugged the ports on the master cylinder, started the car, and the pedal stayed firm and didn't budge. So that isn't the problem.

I then hooked up only the line for the rear brakes (the forward port on the master cylinder, which I believe is for the rear brakes). The pedal stayed firm (car running).

I then hooked up the front also. The pedal quickly sank (car running).

I kept the front hooked up, and plugged the port for the rear brakes. The pedal sank (car running).

So I've determined that the problem is with the front brakes, downstream of the master cylinder (if I have the ports identified correctly, otherwise it is the rear brakes), and significantly worse when the car is running (I'm guessing because of the power-assist).

I have no external leaks.

What am I missing? What can cause pedal-sink other than a faulty master cylinder? Could it be something with the ABS unit?

Just as a reminder, I ran the rear reservoir dry when bleeding the first time (in case that is relevant).
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