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Mark
Mark, Auto Service Technician
Category: Ford
Satisfied Customers: 1504
Experience:  Qualified City & Guilds
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Ford Escort MK 3 1984/5. Removed servo, tested OK, refitted.

Customer Question

Ford Escort MK 3 1984/5. Removed servo, tested OK, refitted. Removed master cylinder from servo - did not break into the hydrolouics. Replaced servo, reconnected master cylinder as a matter of course, bled the system keeping in mind the system was NOT opened. So in my mind no air would get in, making a sealed system. But on bleeding there is a lot of air bubbles out of all nipples around the car, having tested the brake pedal for firmness, but seems soft with and without the engine running. Repeated this several times but still air in the system. How is it getting in there in the first place as it was sealed? The major question is how is the air getting into the system?
Please enlighten me. It goes against all my understanding, since there are no leaks anywhere. I have used at least one litre of break fluid with an easy bled pressure method.
Submitted: 4 years ago.
Category: Ford
Expert:  Mark replied 4 years ago.

Mark :

Hello. My name is Mark and I will be assisting you today.

Mark :

This sounds like you have a faulty master cylinder.

Mark :

Air is probably entering it at the rear seal where the push rod enters it.

JACUSTOMER-3dkvhzul- :

Thank you for your reply.

JACUSTOMER-3dkvhzul- :

Thank you for your rel

JACUSTOMER-3dkvhzul- :

Thank you for your reply and appreciate the concept. But can you enlighten me more on how the air is entering pass the seal besides, it being worn? Where in the piston stroke does the air actually enter the system - what is the process that allows the air to enter? Is it purely the piston going backwards and forwards? If so at what point is the air being taken into the system?

JACUSTOMER-3dkvhzul- :

What is the process allowing the air to enter? Is it happening at atmostphere pressure or is it being pressurised some how?

JACUSTOMER-3dkvhzul- :

Thank you.

Mark :

The rear seal will most likely be worn at the point where the push rod enters.

Mark :

The thing to imagine here is how a servo works. On one side of the membrane is atmospheric pressure and the other side is lower than atmospheric pressure. (Under vacuum). However vacuum doe not mean there is no air inside it. It is only a pressure differential. When you press the brake, it is this pressure differential that gives you the brake assist. ie. it is only atmospheric pressure acting against the lower pressure. Any air in the lower side is normally pushing against the rear seal but doesn't get past it. If the seal is worn then this air can enter the master cylinder.

Mark :

If you look at the diagram you can see the chamber where the push rod enters the master cylinder.

JACUSTOMER-3dkvhzul- :

Having bled the system and pushed all air bubbles out, and accepting there is no air in the system and do not use the pedal to test pressure, will air enter or will air only enter after having tested at the pedal? Even so, which ever is done no pedal or pedal and no fluid escapes from rear seal, when is the air entering? Is the air entering on the forward stroke or on the rerurn stroke

JACUSTOMER-3dkvhzul- :

Having bled the system and pushed all air bubbles out and accept there is no air in the system, and do not use the pedal to test pressure, will air enter, or will air only enter after having tested at the pedal, even so which ever is done no pedal or pedal and no fluid escapes from rear seal, when is the air entering? Is the air entering on the forward stroke or on the return stroke as in brake on or brake off? Is this when air would enter even though there is no fluid escaping from the rear seal? Thank you.

JACUSTOMER-3dkvhzul- :

Remembering there is equal volumne of air out of all four corners.

Mark :

Air probably only enters on the forward stroke. Have you tried clamping all four brake flexi hoses to see if the pedal goes hard. If it does then fault is after the clamps. If not then fault is before the clamps.