Brake servo

Devonrod · Post by **Devonrod** » Sat Aug 08, 2009 8:30 pm

Since a brake upgrade, I need to fit a 1 inch wilwood master cylinder to move more fluid than my exsiting 3/4 m/cyl, as the pedal travel is too long, the lockheed remote servo I have at the moment has a 5/8 bore, what I want to know is what servo will work with the 1" m/cyl ?, it also has to be a 7 inch unit I have no room for an 8 inch.

I dont want guesses I want someone who knows!

And before you start Ian Stewart ,yes I am a girl, yes I am an old git, I need a servo ! , you can stop the brakes alright in the cortina but you are 6ft 6in and heavy !!!

Chart below -

Thanks
Dave

davemgb · Post by **davemgb** » Sat Aug 15, 2009 1:48 pm

Can't say I know for sure but....

Your current servo will work with the new master cylinder, if my understanding is correct the servo flows fluid freely until some pressure is developed in the system at which point the servo senses the pressure rise and adds its contribution to your effort.

Since you have not said what else has been done to the brakes other than the m/c change it is difficult to say what effect there has been on the effort needed on the brake pedal but if you have driven the vehicle then you should already know if you want more assistance or less and can select a servo with more or less boost than your current one. A word of warning, can you lock the breaks if you try - if yes then I don't think you don't need any more servo assistance.

Dave

ian.stewart · Post by **ian.stewart** » Sat Aug 15, 2009 3:37 pm

YER BIG WUSS

ian.stewart · Post by **ian.stewart** » Sat Aug 15, 2009 3:52 pm

Dave,
I thought Kapri on the NSRA site was the in house brake man

Post by **Coops** » Sat Aug 15, 2009 5:10 pm

could always speak to CSR on here,
he helps out at caprisport on there braking systems and maybe able to help out.

Devonrod · Post by **Devonrod** » Sun Aug 16, 2009 10:32 am

Thanks for the replys.

Ian, the plot thickens!
I just took apart the 8" servo that packed up 20 odd years ago when you gave me that 7" one, after I put on your 7" one I had to change the 7/8 master for a 3/4 one, yet I find the old 8" is a 5/8 bore same as the 7" !!!
I dont understand these blooming things!

I have Kapris opinion, I just would like more input as servos are expensive as you know and I dont want to buy the wrong one.

ian.stewart · Post by **ian.stewart** » Sun Aug 16, 2009 8:24 pm

Dave, is there any reason why you could not mount a normal master cylinder on a servo, they must be similar in operation as I use a cylinder off a servo mounted on my bulkhead.

Devonrod · Post by **Devonrod** » Mon Aug 17, 2009 7:32 pm

I think the only to do Ian is try what ive got, I emailed Wilwood but as usual the yanks ignore any mail from the UK !!!
Denis has found some cheaper cylinders than Wilwood anyway, so I will end up with one of them.

unstable load · Post by **unstable load** » Tue Aug 18, 2009 1:11 am

Devonrod,

I am sure I am preaching to the choir here, but the diameter of a booster has no relationship to the diameter of the master cylinder.

The booster uses engine vacuum over the diaphragm area to create a force that acts on the M/Cyl and then on to the wheel cylinders. The bigger the booster the more force is applied to the master cylinder.
If you fit a bigger master to an existing setup then you will in effect need MORE force to actuate the brakes to the same system pressure due to the ratios between the cylinders.
If you want to use less pedal force to brake then you either need a SMALLER diameter master or a BIGGER booster.

If you have the space available you could fit a second booster into the system and that should lower the required pedal force to a more comfortable level. It can be anywhere in the car like the boot for example as long as it has access to engine manifold vacuum.

Devonrod · Post by **Devonrod** » Wed Aug 26, 2009 5:51 pm

Unstable load, thanks for the reply, I am aware of all you say, I cannot fit another servo as I barely have room for the 7" one I have, as you will see from the chart in the first post 7" Servos come in sizes from 5/8 to 7/8,what I want to know is what difference to the performance does that make?

unstable load · Post by **unstable load** » Sun Aug 30, 2009 2:14 pm

Devonrod,
I am sure there are folks on here who are more up to speed with the maths involved to calculate the forces involved.
You will need to know how much the manifold vacuum is at the servo and with that you can use the diameter of the diaphragm to calculate the force the servo will deliver to the master cylinder regardless of the master cylinder diameter.

As the booster only assists you by lessening the pedal forces, it should be relatively easy (for someone with better maths skills than me) to calculate the available pressure with a set pedal pressure and the different cylinders.

This may help you with some of it... http://www.hotrodheaven.com/tech/brakes/brakes4.htm

Good luck, mate!

Devonrod · Post by **Devonrod** » Sun Apr 28, 2013 3:20 pm

I thought I would bump this again in case there are any new thoughts, I put on a 1 inch pattern Land Rover m/cyl and kept my 7 inch 2.3 ratio servo, the brakes stop fine but they just dont feel right !?
I now have a 1 inch Wilwood m/cyl to go on, as I had a few issues with the Land Rover one and before I buy a new servo I wish someone out there could tell me what to use! ive had loads of suggestions but they contradict each other !
Dave

DEVONMAN · Post by **DEVONMAN** » Sun Apr 28, 2013 6:13 pm

Dave,
I think you need to go back to brass tacks and first stablish the the following.
1. The diameter of the front Wilwood 4 pot caliper pistons. (8 total)
2. The diameter of the rear Granada caliper pistons. (2 total)

This will enable calculation of the volume of fluid to be moved when you apply the brakes.

This volume needs to be less than the full stroke volume of the servo cylinder.
If it isn't less, then the servo will bottom out before the brakes are fully applied and loose it's effect. The slightest bit of air in the system between the servo and the callipers will add to the problem.

Compared to my car you have quite large calliper pistons and the 5/8" bore of the servo may be marginal to shift the required amount of fluid. Just like if you did not have a servo and a 5/8" master cylinder, then I think the pedal would go to the floor and bottom out the master cylinder.

I have smaller calliper pistons than you all round and a 7/8" bore servo (1.65 ratio) and mine is fine and feels good with a 7/8" Master cylinder.

It's a guess but I would imagine calliper pistons move about 10 thou when the brakes are applies given the likely knock back from wheel bearing slack etc. but someone else on here may be able to put a better guess on this.
Once this is established then we can attempt the maths.

However, If the master cylinder is sized correctly you should in any case have a firm pedal with the engine off and no vacuum in the servo. Start the engine and the pedal should move down slightly.

Hopefully you will get some new inspiration from someone on here.

Denis

Devonrod · Post by **Devonrod** » Mon Apr 29, 2013 1:49 pm

Denis, the front Wilwood pistons are 1.62, I dont know what the granada scorpio (vented) piston size is, anyone ideas ?

ian.stewart · Post by **ian.stewart** » Mon Apr 29, 2013 2:34 pm

42.8mm