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MG MGB Technical - Servo orientation

I am fitting a servo with the air valve pointing up which I believe is according with the instruction manual, image 1. But I have also found a photo which shows a unit mounted with the air valve pointing down, image 2. Which is correct? Does it matter? And if so why?





Graham Gilmore

Originally they were fitted with the air valve up and virtually level, aftermarket Lockheed instruction are down and at an angle, however I don't think you will get that angle in an MGB as there is so little space between the heater shelf and the bonnet.

Reasons given have been that it lubricates the air-valve piston to prevent the servo sticking on, and makes bleeding easier. But others have found the angle of the air-valve makes no difference, nor lubrication in the upper position, only polishing the bore with fine wet and dry. I have one in the roadster which does tend to stick at slow speeds in hot weather, but the one in the V8 never has. OTOH a pal's V8 stuck so badly on a hot trip we had to disconnect the servo hose and plug it, which tends to confirm it's down to the individual servo.

As far as bleeding goes an upward angle will trap less air in the servo cylinder, but there will still be some. On both cars bleed is a two-stage process - the basic fill and bleed using a gunson's, then a high-pressure bleed with someone pressing the pedal hard while I rapidly open and shut each caliper nipple in turn which always blasts an extra 'lump' of air out after which they are fine. Maybe a greater upward angle would help, but the rare occasions bleeding is needed it's not worth messing with the mounting and pipework.

If you are fitting one to a car that hasn't had one before, and have the standard in-line servo, it makes very little difference. I've driven a pal's car without and didn't even notice, and although I was specifically looking for the difference on my pal's V8 it was barely there. There are servos with a higher ratio but what the drum size is in terms of fitting it in the usual place ion an MGB I don't know.



paulh4

CORRECTION - the pictures numbers were transposed in my original posting. The test should read

I am fitting a servo with the air valve pointing down which I believe is according with the instruction manual, image 1. But I have also found a photo which shows a unit mounted with the air valve pointing up, image 2. Which is correct? Does it matter? And if so why?

Graham Gilmore

Same response.
paulh4

The problem I always assumed was bleeding the air valve piston.
Because of the space issues involved in air valve down, I put a bleed nipple on the end of the elevated servo cylinder. It seems to work better than horizontal cylinder, valve up!


Allan Reeling

I'm with Paul, on a chrome UK car with single circuit brakes it makes little or no difference. My 1971 B had excellent brakes, no servo and without a heavy pedal. Why add complication? The servo makes bleeding more tricky, and is a known failure point as they age.
Mike Howlett

A servo doesn't improve braking a single jot, it just reduces pedal pressure. Ultimately maximum braking is determined by the grip of the tyres, and without a servo the B can be brought to the verge of brake lock where grip is at maximum. For better brakes, fit better tyres. (If mountain passes at pace are your thing then upgraded linings/pads will be needed if you are a hard charger).

All that said, there is a good reason to fit a servo if the pedal pressure differs much from other cars you drive and you find it disconcerting or difficult to adjust to.
Paul Walbran

I wish I could disable the servo on my Mercedes - or at least reduce it, far too fierce.
paulh4

This thread was discussed between 13/11/2023 and 18/11/2023

MG MGB Technical index

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