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Ford Cortina Brake Servo


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My S7 is based on a 1981 2000cc Cortina Estate and was completed in 1995.  The other day whilst driving I touched the brake lightly and the brakes came on full.  Releasing the brake quickly released the brake.  Whilst sitting stationary with the engine idling, gently pressing the brake causes the brakes to engage suddenly and the engine note changes suggesting air leaking back into the servo vacuum tube.  If I disconnect the brake servo vacuum hose and drive without the servo assist, the brakes work without snatching but I would not be comfortable driving this car without a servo.

My conclusion is that the seals in the brake servo have failed and a slight movement of the brake rod causes uncontrolled air to enter the servo, with the vacuum then pulling hard on the diaphragm to operate the brakes.  The Haynes manual for the Cortina states the servo can not be serviced but is likely to last the life of the car. (40 years?) If there is a problem, obtain a replacement unit.  As I have discovered in my internet searches, there does not seem to be a company offering a repair service or suitable direct replacement.

Can anyone suggest a solution to this problem, perhaps an alternative I could use, ideally one that would take my master cylinder so I would not have to re-route the brake pipes.  The Ford Fiesta looks a possibility, although the brake pedal rod looks shorter and I don’t know the dimensions of the master cylinder mounting and if my master cylinder push rod would be compatible.

Perhaps someone might have an old Cortina brake servo in their garage that they could pass on to me.






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Thanks Dave, but no luck there.

On Wednesday I visited a car breakers yard that had over 300 cars on display for a help yourself spare parts service.  I didn’t know that sort of place existed anymore! I spent a couple of hours looking through about 100 cars for a brake servo that had about the same diameter and depth as the Cortina one with a master cylinder of similar diameter and length with 2 outlets for the front brake lines and one for the rear.  The best match was one on a Mazda Bongo van and the second best was the Mazda MX5.  Unfortunately, the plastic reservoir for the hydraulic fluid on all the cars had been disposed of along with the fluid.

However, I managed to find a complete unit on eBay and have ordered this.  I will need to lengthen the push rod between brake pedal and servo and make a new bracket to mount it. Then, my plan is to see if the Cortina master cylinder fits the Mazda servo.  If so, that will make that install a lot easier.  Otherwise, it will be a case of re-routing the brake lines and possibly modifying the reservoir if it is too high for the bonnet.  Then fingers crossed that the hydraulic ratio is close enough to make the brakes work.

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The body of the Cortina master cylinder does fit the studs on the Mazda servo but the push rod coming out of the servo is too long.  It protrudes about 0.6” whereas the Cortina one is about flush.  I considered cutting it shorter but didn’t want to risk damaging the servo so decided to make a spacer between the 2 units.  However, the studs on the servo would then be too short to hold both the spacer and master cylinder. So, the spacer will have new mounting studs for the master cylinder and I will mount the servo to the space at 90° using its own studs.  Anyway, that’s the plan.

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Have you looked at the servo from a Sierra? I'd be surprised if its massively different to the cortina.

My servo and master cylinder look a lot like your setup.

Ford are pretty lazy at redesigning stuff, often re-using bits from a previous model. 

I understand there may not be many around, but it might be a simpler fix.

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  • 4 weeks later...

Well, I finished the conversion a couple of weeks ago.  There was a great deal of measuring and trial fitting.  The brackets were made from 2 thicknesses of 20 x 2mm mild steel bar plus some aluminium angle – this was to allow adjustment of the servo to the correct position and angle.  The spacer was a sandwich of 5 pieces of 3mm aluminium. I lengthened the push rod from the brake pedal to servo with 2 pieces of 3mm flat mild steel plate, cut in a curve to match the Cortina one.

I also had to reposition the clutch cable and radiator header tank and add a vacuum one-way valve.

The set up worked but the servo is much too powerful.  It only needs a slight touch on the brake pedal to cause the brakes to operate and a gentle push to lock them up.  So, the car is drivable but a little scary.  I thought that using a brake servo of the same physical size as the Cortina would give similar braking, however that is obviously wrong.  Perhaps modern servos are more efficient and I will need one that is physically smaller than the Cortina.  I plan to choose another and repeat the fitting process!




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It is not the size of the servo that dictates its force gain. The size gives the maximum force.

Inside the servo is a proportioning valve that sets the gain.

There are 2 springs that set the gain, The only time I took one to bits I found that the springs had a lot of stored energy (as I expected) so precautions during disassembly need to be taken.


Is a video that gives some idea of how these work.

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I am not certain how to choose the servo as I have not seen a list of servo gains.

There is another variable which is the size of the master cylinder bore. A bigger bore will give a lower pressure and information on bore sizes is available.

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