Another brake question...

[Boomlander]

Hmmmm, I think I will get the braking system checked at the local Classic car workshop and see what they think.
The car did pass it's MOT test in February but it won't hurt to get a second opinion.

[geoberni]

The one I fitted to ours didn't specify which way up the atmospheric air inlet valve goes but was very specific that the hydraulic outlet must point up ( I think it was around 30 degrees). It bleeds easily like this.

This is basically what I have, interestingly it is mounted with the air valve on the top. So it seems for some the valve on top is OK.

https://www.carbuilder.com/uk/remote-brake-servo

Well essentially, having the Air Valve downwards is a common installation error, indicating that people don't understand how it works. The error is repeated by some resellers and fabricators. It's not so much as a 'wrong way' to fit it as a 'very poor way' to fit it.

See this long write up on the AP/Lockheed servo and how it works in practice.
http://www.mgb-stuff.org.uk/servo.htm
Almost at the end of it you will find it says:

Image from AP/Lockheed fitting instructions (this link downloads a PDF) found on a Lotus Elan forum showing a pronounced upward angle of the cylinder, and the air-valve assembly pointing downwards, both of which will reduce the amount of air that can be trapped in the servo.

It should be obvious to anyone with the slightest knowledge of the behaviour of air in liquid, i.e. knowing that air rises to the highest point, that with the Air Valve uppermost then some air will be caught in the point circled, even if the correct overall nose high position is achieved.
It's only a little, but it's there.... When you consider that some people have previously written in this 'ere Forum, that they found an improvement in their braking by eliminating the air trapped in a replacement Brake Pressure Switch, given the overall volume of the Minor hydraulic System, compare that to the air trapped in a poorly orientated servo.... I'd guess at least twice as much as a pressure switch.

Servo 1.JPG (70.87 KiB) Viewed 1294 times

So it's all very well manufacturers/suppliers saying tilt the whole thing Nose High, such as on your link where they say

The unit can be mounted at any angle but please bear in mind that there is no bleed facility
on the cylinder so it makes sense to keep the outlet higher than the inlet.

But to me, at best, that indicates a bit of a blasé attitude to the subject of correctly bleeding brakes.

Tilted 30 deg Nose Up with the Valve on top you have this un-bleedable air trapped....

Servo 2.JPG (66.53 KiB) Viewed 1294 times

Simple as that...

[geoberni]

Hmmmm, I think I will get the braking system checked at the local Classic car workshop and see what they think.
The car did pass it's MOT test in February but it won't hurt to get a second opinion.

Well I've given you the explanation, with the appropriate illustrations to assist in understanding it, so if your 'local Classic car workshop' say it doesn't matter, ask them to explain why; it might be a valuable insight into their actual knowledge base. They might be the Bee's Knees at Classic Bodywork, but know naff all about basic school child Physics ...

[kevin s]

I have plenty of understanding of automotive engineering, infact I have a degree and a Msc in it, you are obviously talking about a particular design of servo, others may be of different designs which don't trap pockets of air, cbs have probably sold hundreds of those servos for many years and not changed their advice or mounting which would lead me to think there is not a generic problem with them not bleeding.

[geoberni]

I have plenty of understanding of automotive engineering, infact I have a degree and a Msc in it, you are obviously talking about a particular design of servo, others may be of different designs which don't trap pockets of air, cbs have probably sold hundreds of those servos for many years and not changed their advice or mounting which would lead me to think there is not a generic problem with them not bleeding.

Ohhhh.... Get you...
I've know many graduate engineers over my years, including MScs, some were great people, some were OK types, while others were complete imbeciles when it came to basics, who I wouldn't trust to change a fuse in a BS 1363 3 pin plug!
The mere fact that you feel the need to mention it speaks volumes.
So sorry, not impressed by the Post Nominals.
The facts speak for themselves.
The end effect of that amount of trapped air may or may not be of concern to some people, but to deny that it is there, is foolish.

I've posted up the sectioned drawings.
Many of these Servos are direct copies of the AP/Lockheed one and they made it clear how it should be fitted.
The CBS one from its external appearance is identical to it, just as many are.

There are a great many Fitment Instruction Leaflet PDFs that can be found which all state the same:

The air control valve should be situated at least 30° below the centre line to assist when bleeding the hydraulic system.

Your faith in CBS, without any evidence is no recommendation.
I have no reason to criticise CBS directly, I've not seen their actual fitment instructions, but if it looks like a duck and sounds like a duck, it's very likely a Duck.

I'll let people make up their own minds.

[chrischris59]

CBS fitting instructions support your stance. https://www.carbuilder.com/uk/remote-brake-servo

[kevin s]

The point I was making was that any servo bought is roughly the last 20 years won't be a genuine Lockhead one, it will be a far eastern copy that may look similar but will be of their own design internally possibly to reduce cost but also to maximise its use across the widest possible number of applications. Making the design such that control valve port self bleeds would not be difficult if one wished to do so.

My advice to anyone with a Servo would be if you have a firm pedal there is no reason to change anything, if you have a spongy pedal firstly make sure the fluid outlet is higher than the inlet, they all seem to agree on this, if you still have problems then try turning it up the other way.

Kevin