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Hello, I am in the process of fixing a Renault Scenic 2001 1.9dci.
Before you ask I know what I'm doing and I'm a qualified mechanic, but I've ran in to a problem that I've never before experienced, even my cousin who has over 20 years of experience never seen this before and we cannot figure it out. So I'm asking here if anyone has any clues.

So it all began when me and my cousin spotted that one brake pipe is broken and leaking, so we decided to change the pipes. I took the pipes off and since it was too late and shops were closed we left the brake system open throughout the night, and as you would assume all the brake fluid ran out of it through the remaining healthy pipes.
Next day, I installed the new pipes in to place and we tried to bleed the system of air. While one rear brake caliper was building up pressure the other one wasn't. We checked the pipes, for any leaks or damage or even clogs, but found nothing. We tried to bleed the system with the engine on, because we though maybe the ABS is not building up pressure since it's off. But the problem persisted.
So we turned off the engine and looked deeper. We checked if the pressure was coming out of the ABS block, it wasn't. So then we checked if the pressure was coming in to the ABS block, it wasn't. So then it was pretty clear, it must be the cylinder at fault.
We took out the cylinder and then took it apart, but to our surprise everything seemed normal, no damage, no dirt, nothing. We had another cylinder from an English car, we took that apart for comparison, all seemed in order. So we cleaned it and put it back together. When I installed the cylinder things got worse.
The brake pedal was empty and no amount of trying to bleed the system helped to restore the pressure in the brakes. Okay we though maybe I didn't insert the cylinder right, but there is not much you can miss, it inserts very easily. So we were left confused.
We kept trying to build up pressure in the system to no avail. We tried bleeding the front brakes even though we didn't touched them, no pressure there either, not in the ABS block, not in the brake calipers, nowhere.
When my cousin took off the pipe from the cylinder and put his thumb on the hole I could feel pressure in the pedal, which meant that the cylinder is building up pressure, but when we put the pipe back on, no amount of pedal pumping builds up pressure, even though the brakes system is sealed shut.

So there it is, we are at a complete loss. If anyone has any ideas as to what might be the problem I'd be very grateful if you could share them.
 

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Try this first, before condemning The ABS pump,. Fill the reservoir, now undo all 4 top pipes from the reservoir/master cylinder to the ABS pump.

Wait until you see fluid appear at all 4 unions, Dont tighten them up.
Now pump the pedal SLOWLY, as slow as possible, once you see the air bubbles vanish, close these 4 pipes, should take 6 pumps to complete this..

Now with a syringe and a piece of tube,, (You will need one to make your life easy, )
Go to the Back Near Side Rear, Or the side with the pipe/calliper furthest away from the ABS pump, put the pipe on, connect the syringe, and pull the fluid out by pulling on the syringe Plunger, you will have to do this until you get fluid out, You will need to take the syringe off a few times, from the tube, to press the plunger back in,
DONT now get tempted to use the brake pedal, until you can get fluid out of all 4 wheels using the syringe..
larger capacity the syringe holds the easier it will be...
Unless you have a hand primer pump floating about , then you could use this..
But you will need to pull fluid through,..
Otherwise its connecting up to diagnostic machine and allow bleeding through the ABS system..

Next time you ever have to do a brake pipe on a car..
Undo the brake pipe end, closest to the calliper . Now hold your brake pedal down,, A bar from the drivers seat down to the pedal works well, you can pull the seat forward..
You can now leave this for as long as you want. No more fluid will come out, and you will only have to bleed to the pipe end that you have replaced..

You wont have to bleed off the calliper, or the wheel cylinder, as no air can get in, and no fluid will come out, until you request it by pressing the pedal..
Just make sure your Brake lights are not on, when Ignition is off and key in your hand, This needs checking first, (if no radio code as in different cars, and Instead of disconnecting the battery, customers get reallllllly pi555y when you dont have your radio code calculator with you and they dont have the code, DAMHIK) then disconnect Brake pedal switch, and put a note on the dash for yourself ..
 
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