Finally got around to diagnosing the problem - it was caused by stuck valves in the ABS hydraulic block. The block has 2 valves for each brake caliper (8 valves total) plus 4 more valves related to traction and stability control.

The valves for each caliper are:

1. Inlet valve that is normally open, permitting fluid to pass to the caliper.

2. Outlet or dump valve that is normally closed. It opens when commanded by the ABS controller to reduce line pressure by dumping excess fluid to an accumulator and the ABS pump inlet.

In my case the normally open inlet valves for the RF and LR calipers were stuck closed, preventing fluid flow from the master cylinder to the associated calipers. The hydraulic block is a sealed unit that has little possibility of repair without specialized tooling. Xemodex in Canada sells remanufactured blocks for several hundred dollars. For $75 I obtained a used block from a local auto recycler and confirmed that all 12 valves were working before installing the block.

When the ABS controller/hydraulic unit is installed in a car, the valves (and pump) can be individually activated by a Vida/Dice system. A clicking noise will be heard if the valve being activated is not stuck. That is how I originally diagnosed the problem, after realizing that no fluid was flowing during attempted caliper bleeding.

To test the valve block on a workbench, I separated the ABS controller electronics module from the hydraulic block and used a solenoid coil similar to the coils in the ABS controller. An equivalent test setup could be created by removing a solenoid coil from a defunct ABS controller module and connecting the coil through a pushbutton switch to a 12v battery or power supply. Then a candidate hydraulic block could be quickly tested by placing the solenoid coil over each projecting valve sleeve and pulsing power to the coil. Clicking noise = valve OK, silence = stuck valve.

Depending on which valves are stuck, it is possible for a hydraulic block with seized valves to seem OK under normal non-ABS braking. However a stop with ABS activation would cause the brake circuit with seized valve to behave improperly.

Perhaps the valves in my original hydraulic block seized because the brake fluid was not flushed often enough by the previous owner. After all the work it took to understand and fix the problem I have incentive to regularly flush the brake lines.

Mike

I'm late to this threaad.

I agree that you should start with a 4 wheel brake bleed.

How is the brake peddle? is it soft, or does it drop down slowly when you step on it? It should be firm.

Who installed the new Brake MC? Don't assume it's OK.
It first needs bench bleeding.
Was it a new OEM unit or a remanufactured?
Beware remanufactured MC's many are junk since all that some do is replace seals. If the bore is corroded or worn it needs machining or a new sleeve installed. My sense is that with a MC, you bite the bullet and buy only new OEM. You might luck out at Ebay and find a good price.

Another thing can be shot flex brake lines. I'd replace with new rubber
and not stainless, since stainless invites electrolysis nearby.

Good luck, Bill

The easiest thing to do would be to bleed the brakes, all 4 of them. Make sure you have the proper fluid for 2006.

If that doesn't fix it, suspect the caliper and take a good look at the slides.

When I bought my 850, previous owner must have complained about noisy brakes. The pads were glued to the brake pistons with red RTV, along with glue on the pad hangers - the pads couldn't move! Of course the brakes didn't work and needed 2 new rear calipers, along with rotors.


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Keeping it running is better than buying new

“First check will be the calipers to make sure they arent binding, then confirm that the pistons do move when brake pedal is pushed. Bleeding all calipers would probably be the next step.”

That is exactly where I would start. I suspect that there might be air in the RF/LR hydraulic circuit. Perhaps the clutch master cylinder repair might have introduced the air and it wasn’t properly bled out?? Having a schematic of the brake/clutch hydraulic system would be helpful.

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Will I buy another Volvo??? We'll see....