Fitting a Brake Servo

[Andysdad]

Hi,
I have a '71 traveller that already has (ford) discs fitted. I am thinking of fitting a remote servo to reduce the pressure needed, in braking. The kits available seem rather expensive for what they are, so I am thinking of buying the servo and vaccumn take-off separately.
My question is (finally got there) about the plumbing arrangement. Does anyone have a schematic diagram or a description of the new pipe layout.
Also what are your opinions on only using the servo on the front and leaving the back alone?

[Declan_Burns]

Here's the schematic that comes with the kit. I have marked up the fittings UNF/BSF.
Regards
Declan[frame][/frame]

[bmcecosse]

I imagine using the servo only on the front - will just lead to dangerous front wheel lock-up, since they will be doing all the braking.......

[beero]

or this may happen..........

http://www.youtube.com/watch?v=dU966ADlVIg

[bmcecosse]

Excellent! He obviously has a hydraulic handbrake on the rear wheels - and no footbrake connection to the rears!

[Andysdad]

Thank you to Declan for the schematic that is exactly what I wanted. I knew that I shouldn't have mentioned only using the servo on the front as it just provided grounds for lateral comments. My thinking was that the front pipe could be plumbed into the servo leaving the rear pipe to feed the back. Thus leaving the rears as normal and servo assistance on the harder front pads.
As I am not an engineer, I just wondered if anyone had tried it.

[bmcecosse]

The rear wheel braking is marginal at best - so the servo will be required on all 4 to try to retain some balance.

[Andysdad]

Thanks you bmcecosse. That's good to know.

[ampwhu]

been running with a servo on the front for years. never had a problem. car pulls up nice and even with hard braking.

[Andysdad]

Now I'm confused again...

[MarkyB]

Better to do the job right and servo all the brakes.
You only need to plumb a Tee piece into the output of the servo somewhere convenient.

[bmcecosse]

The car with servo only on front brakes, either has very inefficient front brakes - or extreme imbalance front to rear! Can it lock all 4 wheels in a crash stop?

[Sleeper]

Updating an old thread...

If discs are fitted to the front and original drums to the rear , can the rears be plumbed into the master cylinder and the front discs plumbed into the servo ?

John ;-)

[oliver90owner]

No.

Well you could, but it would not be safe. Consider the situation where one end is at a hard pedal and the other end is still waving about in mid air!

[Sleeper]

But I thought that the majority of the weight under braking was transferred to the front , hence the question ?
It is only recently ( relatively speaking ) that disc brakes appeared on the front axle, replacing drum brakes , because of that ?

Thanks
John ;-)

[geoberni]

Anything is possible, but being sensible, practical, or practicable, is an entirely different question.
Why would you servo only part of the system?

A Servo on the front and not on the back, would mean pushing harder to get any braking on the rear, which in turn means you're breaking harder on the front, and so on.....

[oliver90owner]

Just think about it sensibly. Braking harder does what? It most certainly does not shift more brake fluid. Liquids are, to all intents and purposes, incompressible.

All you are doing, by braking harder, is increasing the pressure on the trapped volume of fluid.

[jaekl]

It is possible and I recall Detroit sold cars with power assisted front only brakes at one time. However, it's not just a matter of replumbing. The entire system must be designed for the arrangement. It needs to be designed such that the correct amount of fluid will be moved to each wheel and that each wheel will have the appropriate pressure to have a balance braking force. If designed properly, it will provide increasing fluid pressure (braking force) to the front as the vehicle dynamic weight transfers forward due to harder deceleration. Adding a just a servo to a standard Minor brake system doesn't affect the braking balance. It only reduces the leg force to achieve the same line pressure, so as others are saying, simply changing the plumbing will leave the rear cylinders waiting for more pressure to do their share of the job. There are many variables to consider, including the type of front and rear brakes you have and a split circuit master cylinder most likely will be required. I would think the tools necessary to design the system would include line pressure gages and a rolling dynamometer to evaluate the braking force applied by each wheel and many trials.

[kevin s]

I have this on our minor along with a dual circuit master cylinder, (it's advisable to do this to isolate the circuits) if you fit disc brakes you generaly move the bias to the rear as discs don't self servo like the dual leading shoe set up usually used on the front, this could be dangerous you always want the fronts to lock up before the rear, fitting a servo only on the front was also done on some production cars, corsairs being one I am aware of.

Testing ours in a car park (like they do on an IVA test) shows the front locks up just before the rears.

Of course it also depends upon the size of discs type
of pads etc used .

[Sleeper]

I have this on our minor along with a dual circuit master cylinder, (it's advisable to do this to isolate the circuits) if you fit disc brakes you generaly move the bias to the rear as discs don't self servo like the dual leading shoe set up usually used on the front, this could be dangerous you always want the fronts to lock up before the rear, fitting a servo only on the front was also done on some production cars, corsairs being one I am aware of.

Testing ours in a car park (like they do on an IVA test) shows the front locks up just before the rears.

Of course it also depends upon the size of discs type
of pads etc used .

This sounds promising..

Thank you.

John ;-)