Servo bore sizes - thoughts and opinions?

General Chat with an emphasis on BMC Minis & Other iconic cars of the 1960's. Includes information on MK1 Action days.
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AndyPen
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Servo bore sizes - thoughts and opinions?

Post by AndyPen »

I have a few servos here, some have been sat a good while - since the 90s.

I think I only have one with no rings on the nose, and that's an older one. I also have an Italian alloy body one that I picked up at a show for some reason.

Before I go sorting through them / seeing if the one without the rings is restorable, what are the thoughts on the other bore types, is anyone using those?

Probably a post or two on here already but unfortunately my search hasn't found it.


servo diag.jpg
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Andrew1967
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Re: Servo bore sizes - thoughts and opinions?

Post by Andrew1967 »

I’ve used an original one with one or two rings on 9748KO with no problems at all. Very good ‘road’ brakes.
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Re: Servo bore sizes - thoughts and opinions?

Post by mk1 »

Personally, I have never been a fan of any servos on Mini brakes. All they seem to do is reduce the feel you get through the pedal.

I do however know of people who have used various bore servos on Minis & all seem to have worked OK, I imagine they give a bit more or less "help" but all seem to be perfectly acceptable.

Having said the above, I'm probably not the best person to comment as I think they are all rubbish :)
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iain1967s
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Re: Servo bore sizes - thoughts and opinions?

Post by iain1967s »

I’m using the Italian alloy body Bendix/‘Alfa’ servo with the larger bore currently. I noticed that it doesn’t kick in until a higher pedal pressure, compared to the original Iron body smaller bore Lockheed S servo.

So basically at low pedal pressures when you want to slow gradually it’s not providing much/any servo assistance, but when you want hard braking it does reduce pedal effort quite a bit compared to no servo (but it’s still very controllable, it doesn’t lock up the wheels unnecessarily).

But as Mark says, it’s really not necessary. I drove my S for a year with a failed servo bypassed entirely, and it was still perfectly safe and usable, just took a bit more leg work to come to a quick stop.

[driving with a failed servo and NOT bypassing the hydraulic line is an entirely different consideration, the brakes are almost unusable as the pedal has to push against a reduction ratio of the ‘dead’ piston in the servo bore, which makes for very very hard leg work to stop the car]

ps. I have the Special Tuning C-AJJ3388 split circuit front/rear master cylinder arrangement, so the servo is only driving the front brakes per the ST instructions.
AndyPen
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Re: Servo bore sizes - thoughts and opinions?

Post by AndyPen »

Useful perspectives, thank you gents.

Our rally car didn't run one but I've made the conscious decision to put one back in, to try and bring the car back to what it was like in the late 60s, when it still had one. I know I have the original one out of the 970 and a couple I've picked up over the years, mostly in parts. I also found one in my shed as some may recall, with thread seals in - that make it look like a refurb.

Is there any testing that can be done on the bench folks?
Dearg1275
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Re: Servo bore sizes - thoughts and opinions?

Post by Dearg1275 »

I find with the larger bore servo there is more brake pedal travel before the brakes bite. As the bore is larger the master cylinder has to deliver more fluid to move the piston in the servo as compared to the smaller bore. The answer, I suppose is to fit a larger bore master cylinder at the same time. ADO 16 ones perhaps?

D
AndyPen
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Re: Servo bore sizes - thoughts and opinions?

Post by AndyPen »

Dearg1275 wrote: Thu Nov 24, 2022 6:03 pm I find with the larger bore servo there is more brake pedal travel before the brakes bite. As the bore is larger the master cylinder has to deliver more fluid to move the piston in the servo as compared to the smaller bore. The answer, I suppose is to fit a larger bore master cylinder at the same time. ADO 16 ones perhaps?

D
That's an excellent point to consider - thanks. I have a NOS S master already fitted and plan to use the clear extension tops on the cans. A bit more fluid and you can see the level at a glance.
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Re: Servo bore sizes - thoughts and opinions?

Post by mk1 »

Those extension tubes are ace!
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