When is a brake servo required?

[luke g28]

Forgive me if this seems a daft question. I have spent a good while searching but get a million threads on people with broken servos or leaking or similar.

I have a 800 - 1000kg Cortina based kit car that is on track at least once a month.

Do I need a brake servo?

Are systems that don't use a servo designed differently?

If I remove my servo from my current system is this going to be a bad idea or will it just give me a stiffer pedal with more feel?

All feedback welcome

[flemke]

Disconnect servo and try it out. There are things that you can do to compensate for not having servo, although they tend to result in either longer pedal travel or more unsprung weight.

[Sam_68]

flemke said:

Disconnect servo and try it out.

You'll almost certainly find that you need an unacceptably high pedal pressure to get the necessary braking effort, because...

luke g28 said:

Are systems that don't use a servo designed differently?

...yes, they will have their master cylinders sizes accordingly, to give the appropriate level of hydraulic effort.

Of course if you have a car that is vastly lighter than the original (ie. a 500kg 'Seven' using the braking components from a 1200kg donor), you have a fair chance of getting away with it, but at 800-1000 kilos, you car isn't that much lighter than the donor 'tina.

But there's no harm in disconnecting the servo and trying.

[flemke]

Sam_68 said:

flemke said:

Disconnect servo and try it out.

You'll almost certainly find that you need an unacceptably high pedal pressure to get the necessary braking effort, because...

luke g28 said:

Are systems that don't use a servo designed differently?

...yes, they will have their master cylinders sizes accordingly, to give the appropriate level of hydraulic effort.

Of course if you have a car that is vastly lighter than the original (ie. a 500kg 'Seven' using the braking components from a 1200kg donor), you have a fair chance of getting away with it, but at 800-1000 kilos, you car isn't that much lighter than the donor 'tina.

But there's no harm in disconnecting the servo and trying.

Sure, and also affected by leverage ratio of pedal unit. They are whatever they are; both can be changed.
Regardless of piston size and pedal geometry, it's going to come down to the degree of assistance that's brought by servo, necessary pedal pressure in original (kit) form, and, literally, how strong the OP's leg is.
Should take a couple of minutes to do the experiment, which would be a lot simpler and cheaper than the alternatives.

[luke g28]

Thanks for the replies, I realise the obvious answer is to give it a go but the reason for the thread was that during my engine swap it became apparent the current brake servo wont fit in the bay. So for now ill remove the servo and once everything is back together and running again ill see what the state of play is.

At least now I know its worth having a go before I try and buy a new servo that will fit

[jagracer]

With a servo and a much lightened car the brakes may be too fierce and lock the wheels easily.
You could:
1. Try it without the servo and see how it feels.
2. Go to a gym and build the muscles up in your right leg.
3. Fit smaller master cylinders, this requires less pedal pressure but will give longer travel, possibly too much.
4. Alter the pedal ratio to about 6:1
5. A combination of 3 and 4.

[hal 1]

Stop me if i'm talking b*******s but when you dispel all the air from the servo doesn't the pedal stiffen and not travel as far ?

I'm glad i spotted this thread and the answers so far because i'm in the same position with regard to leg room and was thinking of extending the bulkhead and doing away with the servo.

Any more info would be gratefully received

[Sam_68]

hal 1 said:

Stop me if i'm talking b*******s but when you dispel all the air from the servo doesn't the pedal stiffen and not travel as far ?

You're actually not dispelling the air, you're allowing it in (ie. you're dispelling the vacuum created by the engine), but yes, if you disconnect the vacuum hose to the servo (and block up the engine end so that it doesn't upset the engine), they you lose the servo effect and the brakes will perform as thou a servo wasn't fitted (much harder pedal and higher pedal pressures). That's why Flemke and myself suggested the 'try it and see' approach at the top of this thread.

Edited by Sam_68 on Sunday 3rd July 10:18

[hal 1]

Sam_68 said:

You're actually not dispelling the air, you're allowing it it (ie. you're dispelling the vacuum created by the engine), but yes, if you disconnect the vacuum hose to the servo (and block up the engine end so that it doesn't upset the engine), they you lose the servo effect and the brakes will perform as thou a servo wasn't fitted (much harder pedal and higher pedal pressures). That's why Flemke and myself suggested the 'try it and see' approach at the top of this thread.

So unless i had real problems with the amount of pressure needed to apply the brakes i could just leave out the servo.

An easy way to find out would be to do as you say and just disconnect, block the engine end and try it, I'd do this before any structural work was done to the bulkhead just to see if it was worth it.
Thanks

[magpies]

If in the end you need a servo and there is no room in the usual possition it is possible to obtain a remote servo and place it where there is room - I remember fitting one on the underside of the passenger side parcel shelf in an old rally prep mini. Sorry can't remember from what vehicle I got it from.