11 replies to this topic

[superted87]

Hi

I am struggling to decide what to do with my brakes and old brake pedals are harder to come by. I would like to know the exact specs for a non servo pedal, and a servo pedal. They cant be that different and i think i can modify it to work.

If anyone knows why you have to change it then please ellaborate. Surely the non servo one just actuates an upward motion just like a servo type does.

Any and all help would be great thanks!

[Mini-Mad-Craig]

The servo

[tiger99]

They are very different. The motion is forwards, not upwards, on a servo pedal.

I do not recommend that you even think about modifying one, as it is very safety critical, and if it breaks the results may be fatal for you or someone else.

[Dan]

The difference is the size and position of the hole that the rod is fixed to. The servo pedal comes with a push rod riveted onto it, the hole for the rivet is larger than the original clevis pin hole in the older pedals so if you drill it out there will not be a happy marriage with the non-servo push rod. It's also further forward I seem to remember, giving more stroke to the rod. With a non servo cylinder this has the effect of developing full pressure VERY quickly indeed when you apply the pedal. I'm not sure if I still have one of each here to measure, it would be good to nail down the exact difference.

[Big_Adam]

Far as I found, a servo box has a larger hole for the pin ... fixed that with a lump of metal and re-drilled it.

Pedal box was less than £30, everyone seems to want £50+ for a regular pedal box, so sod it, I'll tweak one to work.

Edited by Big_Adam, 14 March 2013 - 07:53 PM.

[MRA]

You are talking about modifying your brakes in an unsafe manner, if you had a fatal crash your car would in all probability be examined, and when the modified parts are found you would then be liable, you could lie and say you bought it like that and then the previous owner would be interviewed and if by chance they had a picture of that area and could therefore lend some credibility to their story you may find yourself in a local prison.

Not mention that you are putting others at risk for a few quid.

This is covered by the "construction and use regulations"

[Ethel]

Are they pedals from single setup pre '76? The disc pedal will give more stroke before hitting the floor.

All pedals for vertical cylinders will put the pushrod in the middle of the hole so it's concentric with the cylinder bore, save for lateral movement by virtue of its arc. If the horizontal cylinder pedal differs, you'd have to look at the location of the axis of the bell crank, as well as the lever lengths, and calculate the vectors. I suspect it'd matter less than it appears if the pedal & bell crank axes and one of the levers are the same.

[Dan]

Are you changing from drums to servo discs or from discs? As shown above there is an even bigger difference between the drum / 7" disc pedal and servo disc pedal.

[superted87]

woah , quite a debate i have started here!!

i i am going from a 95, sprite with servo pedal box, bulkhead mounted regulator and servo with std 8.4's, and i have converted to 7.5 cooper s callipers, yellow tag dual line vertical master and an in line brake regulator to the rears to balance up the braking effort. I have a straight through T piece at the rear and not a regulator so i need to calm them down a bit!

I have removed the clevis pin from the hole and fixed the pushrod to it and i get actuation but the achieving full pressure bit worries me a little. I read about a guy who had a yellow tag servo, on to a late model bulkhead regulator and he said the brakes were very good. Does anyone know anyone running 8.4's on a non servo set up ?? i may throw my old brakes on to get it through the MOT and ready for summer!

Thanks for your input guys.

Would be nice to make up a cad drawing with all the vectors labelled and angles and hole sizes etc. That would put to bed the question once and for all!

[Dan]

Or just measure them...

The age of the regulator will only matter in that they tend to start failing when they are about 15 years old. After the PDWA system was done away with all the regulators were the same whether there was a servo or not.

You must be certain that the clevis pin is a good fit in the pedal, they do not work well if there is any slack in the system and these are obviously a critical component. You really would be best to use the correct pedal, they aren't expensive or hard to swap.

[superted87]

Dan, cheers for that mate. I will make sure it is a better fit in the hole and try running the outputs through the bulkhead regulator.

The problem with measuring them is that i dont have one, hence the ask!!! And they are expensive and rare! Or i would have bought one right away. So often things are labelled as different but similar enough to work after all!

Cheers

[Globule]

Somewhat of a thread revival. I have a pedal box, how can I identify if it is for servo or non-servo? Thanks!