Fitting a modern master cylinder and servo to an old car

[montyjohn]

Hello,
I have a kit car based on an ’82 2.0 Mk5 Cortina.
My master cylinder has three outlets, two for each of the two front calipers, and one for two rear drum brakes.

My master cylinder (m/c) and servo are old and knackered and both need replacing. I believe it’s circa 21mm dia m/c.
It’s much cheaper to buy a servo and m/c from a modern car, like a Mondeo than replace the Cortina one like for like.
Mondeo m/c are circa 25mm dia and I suspect they have bigger servos.

This would give me a much stiffer brake feel with plenty of assistance.
It would also be future proof if I decide to fit bigger calipers or rear discs in the future.

What I don’t understand is what to do about the balance/bias front and back.
Most modern m/c seem to have two outlets. I assume one goes to the front, just need to add a splitter, and one to the rear (my drums are piped up in series).

Will I have an issue with front/back bias?
The kits car has no metering/bias control as far as I can see.
Does it matter if I use a m/c designed for discs all round and using it on front disc / rear drum setup?

[stevieturbo]

you could add a bias valve to the rear brakes if you want.

I used a Subaru master cylinder and servo on my Granada. It was a relatively easy fit, although I do have larger brakes and discs all round. The servo was also slightly smaller than the original one, which was part of the reason I changed. ( Subaru ones were readily available to me, was the main reason I used one )

[GreenV8S]

If the m/c outlets are at different positions along the body of the m/c you probably have a tandem master cylinder with two pistons in series. These would have either one or two outlets per piston. Wha piston has two outlets, the two outlets are identical and are just a convenient way to split the feed between multiple wheels. If you have a single outlet from the piston and split it with a T piece you get exactly the same effect.

As long as you are only upgrading the master cylinder and keep the relationship between m/c pistons and wheel cylinders/calipers the same, you won't alter the line pressure or the brake balance, you will only alter the brake pedal force and travel.

Unless somebody has paid attention to the static brake balance it could be massively wrong. If you don't currently have a bias valve then even if the static balance is right the dynamic balance will be pot luck - it might be right under some conditions but it will be wrong under others. If you make any design changes such as bigger discs, calipers, converting drums to discs this will certainly alter the brake balance. Impossible to say whether that will be for better or worse without knowing what the existing balance is.

[montyjohn]

stevieturbo said:

you could add a bias valve to the rear brakes if you want.

I used a Subaru master .....

Thanks stevieturbo, good to know something similar has been done. Been looking at the Wilwood Boas Valve which seems to give me all the adjustment I need as the system evolves.

GreenV8S said:

If you have a single outlet from the piston and split it with a T piece you get exactly the same effect.

I used a Subaru master .....

This is exactly what I assumed so good to hear it from somewhere else, as it was just an assumption.
The Cortina m/c is a dual cylinder m/c so I'm all good getting a two outlet unit and just splitting it.

Thanks for the confirmation, just a matter of measuring up now to see what fits, to be honest, the kit car has a lot of room so i think I can pick almost anything.

I thinking of getting something like this mk5 Model Servo and m/c Modern and cheap.
It looks pretty deep, any idea if this is a dual os single servo. Just trying to pick something that has more boost than the Cortina one since I'm going with a bigger m/c dia of 25.4mm vs the 21mm Cortina one.

Thanks

[Chris32345]

Don't most modern mnaster cylinders go to the abs unit first and that splits it to the 4 wheels

[montyjohn]

Chris32345 said:

Don't most modern mnaster cylinders go to the abs unit first and that splits it to the 4 wheels

Yes, they have two circuits so so you always have two wheels braking.
Both outputs go to the ABS distribution block.
Instead I would be using the two outputs direct, one for front and one for the rear.

[stevieturbo]

Chris32345 said:

Don't most modern mnaster cylinders go to the abs unit first and that splits it to the 4 wheels

Sort of.

A lot can have 4 pipes in and out. Although the master will still only be divided into two sections, usually of equal bore diameter.

So for a retrofit, up to the user if they wanted a simple front and rear, diagonal or other split.