360 brake calipers blocking after ABS unit swap

Hi guys,

I had a leaking ABS Unit so I swapped with a 2nd hand one, supposedly taken from a car that was damaged in a barn roof collapse, so in working condition.

I must say that before I noticed that my old unit was leaking, I swapped all disks and pads but very hard breaking remained tricky.

Strangely, the previous leak was small and never let air in the system, that's why it was noticable only from under side.

I did not have any lights on dashboard or error codes about braking.

Anyway, I swapped the ABS units yesterday and it was tricky to bleed all air out with the traditionnal bleed procedure, but now the brake pedal is firm but the system remains under pressure, meaning everytime I touch the brake pedal, it brakes but the calipers don't open back totally, and quite quickly it becomes impossible to move unless I open the bleed valves on both sides to release some pressure.

This release of pressure could to be done in both front wheels or both rear wheels, as the system is divided and linked (see picture attached).

But the cure doesn't last : if I press 5 or 6 times, even slightly, it blocks back.

I let the calipers bleeding valves open for an hour, hoping that remaining pressure would go away, but without success.

Next Week-End I'll perform another bleeding using the AUTEL MS908 Pro from a friend, but I don't understand why this pressure problem happens, since the braking system should work even if the ABS Unit is faulty.

Any idea or suggestions ?

 

I doubt that this would happen suddenly after such an Open/Close operation on the system.
I've performed a full bleeding when replacing the disks and pads, everything was normal, and the car is only 15 years old.

 

You mean, pulling the ABS plug ?
I haven't tried that, expecting the car to enter in fault, but I'll try that right away.

The 2nd hand part was unfitted by a pro and had plastic plugs on all 6 fittings, so yes I am assuming it was "clean" inside.
Would a debris hold the pressure on both sides at the same time ?

 

The fact is that during the bleeding process, just after the swap, the contact never was "ON".
Eventually I tried the pedal with contact and engine ON.
I'm not sure if the calipers were blocked before or after I started the car to check that the pedal remained firm.

 

I just performed another test :
without setting the contact ON, I pushed the brake pedal 5 / 6 times and the calipers got into pressure right away.
So t's definitely somthing holding pressure within the unit.

 

Your hypothesis would be that the 4 rubber lines failed simultaneously ?
I think it would be very unlikely, but thanks for the suggestion.
I'll probably look into that after I performed the "electronic" bleeding, as they say in the service manual.

 

Salut Eric,
Yes, I can release the pressure from the MC valves, and I confirm that both valves needs to be opened to free the car totally.
On each valve, around 3cm of fluid goes out into the bleeding tube.

 

I just talked to Eric and we now are quite convinced that the problem comes from the MC that does not allow the pressure to go back into the feeding lines and reservoir, so I guess my option is now to unfit this guy and hope I can clean it up...

Stay tuned and cross fingers for me !

 

While I am waiting for another MC, I opened the supposed faulty one, but now I’m even more confused : it looks in perfect condition to me...

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even the main o-ing looks perfect
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Well, the guy who sold me the unit is absolutely positive that he drove the car before taking it apart, and the brakes were working. He sent me the unit the same day.
The pressure release on the MC showed that the pressure got back to the MC, but not further to the fluid tank.
Why is that, I don't know, but it would make some sense that the MC was faulty and that it doesn't show on inside parts.

We'll know this week, when i'll fit another MC.
Except if it is my bleeding procedure that created an over pressure, but I wonder how that would happen...

 

Eventually I managed to get the brakes working properly, and ther was several problems hiding one's another :

When I performed the "electronic" bleeding with the AUTEL Pro, some dirt was released and it blocked the front right external side of the caliper. I had to remove the small line at bottom of the caliper and clean it.

I replaced the MC but the blocking remained.

During the whole process, with the help of a friend pushing the brake pedal, somehow the brake booster was damaged and that's the reason why the brakes were blocking : the MC could not get back to it's rest position, then let the fluid get back to the tank.

So I had to replace that booster and everything was solved, with both MCs.

Note for later : you can bleed the system pushing on the brake pedal, but never get to the end of the pedal travel !