View Full Version : Brake Servo working?

06-06-01, 10:40 PM
I have a Unique Autocraft Python using the standard brake set up recommended by the kit manufacturer - basically Jag brakes but with a Triumph servo.

Question, how can I tell that the servo is actually working? The brakes have always required a hefty shove on the pedal to make the car stop but then, when you do push hard there is plenty of stopping power. I wonder if the pedal effort is high because:
a) the servo is too small
b) the servo is not working

How can I test and establish what the situation is?

What other servo's do others use in conjunction with the Jag brake set up?


Nigel B

07-06-01, 08:21 AM

It may be crude and some might say dangerous, But disconect servo from vacuum supply and try stopping, if you dont servo was working.If no change get an expert to look at it.

Regards Steve G.

07-06-01, 08:39 AM
If I recall correctly, according to the Haynes manual for another car (fiat uno!) this is how you test if the servo's working:
with the engine off (and car parked) press the pedal a few times until it feels stiff - this gets rid of the vacuum I think. Then with your foot pressing on the pedal start the engine - if the servo's working ok you should feel the pedal drop a little bit.


07-06-01, 09:18 AM

Ken Freeman of AK Sportscars recommends the use of a Marina or Ital servo with the Jaguar system, and preferably an Ital van item due to its greater power. Also possible to use the Sierra item.


07-06-01, 11:55 PM
I have used MGB servo on SRV8 (recommended at the time) and work adequately, later cars used Ford Fiesta servo and although sounds wimp actually is ideal and works well and is cheap.

09-06-01, 07:51 PM
I have a brake servo and master cylinder thats been overhauled from a Marina. This is what AK use and I was going to use it, but I decided to build a Dax instead. If its of any use to you I could send it to you if you pay for postage. They use it with the Jaguar brakes and its supposed to be a very good match. philip@pringwood.freeserve.co.uk

20-06-01, 12:03 PM
Hi Nigel,

This may be way more of an answer than you expected or require but here goes anyway.

I don't know if you have this sorted now but you re getting into a really complex area. Basically there are a number of things to consider when it comes to the braking system and the need or otherwise for a servo. Manufacturers fit them because it is a cheap cure all solution saving them the cost of manufacturing braking systems specific to each model.

1) If your engine is running a wild cam it will produce little or no vacuum in the first place so your servo will not operate. You can install an electric vacuum pump but if your electrics fail so do your brakes!!
2) There is a requirement to calculate the amount of pressure needed to push the caliper pistons. This will indicate the size of master cylinder/pedal ratio combination. A small diameter master cylinder will produce a greater pressure, reducing the need for a servo, but the volume will be down. Once your master cylinder has moved it's full volume you can press as hard as you like but you will not stop any quicker. A larger cylinder will move more volume but the pressure is down resulting in a harder pedal, lower braking efficiency and the need for a servo to boost it.
3) Be carefull where the master cylinder comes from. If the donor vehicle used drums at the rear you may have a residual pressure valve in there which will prevent the rear pads from fully retracting in a disc all round situation.
4) The pedal ratio is an important determining factor for brake efficiency, a 6:1 ratio will exert about 25% (this is all from memory so bear with me) more than a 5:1 which is quite considerable in terms of actual pad to disc pressure.
5) Pad material also makes a deal of difference, I recently changed from Mintex 1144 pads, in my own Jag based AP setup, to standard material pads and picked up a lot of pedal feel and no need to warm the brakes before they become effective.
6)Re checking your existing servo, any garage offering Sun or Crypton tune ups will be able to measure the vacuum your motor is producing at various rev levels. The push pedal start engine tip offered above then a good method of checking the unit itself.
7) As you are using a system designed for a vehicle of different weight, power, wheel/tyre combinations and the majority (70%+/-) of your braking is done on the front it may be wise to install a bias valve to assist with fine tuning once complete and installed.

I can post a full break down of all this incredibly boring bedtime reading if you wish. It is really helpfull and sheds some light on one of the more mystical areas of our hobby.

One final thing to consider........

Race cars do not use servos.