New brake servo

[Marlindan1965]

Firstly Happy New to Everyone.

As the weather is still very mild in Gloucestershire I was hoping to take the Roadster out for spin. However as I manoeuvred the car out of the garage I realised I had no break pedal. I checked the brake fluid reserviour to find that the level had dropped right down.

I investigated for any leaks but didn't find any obvious signs of leaking. So in discussion with my brother in law, who is much more knowledgeable than I am, he suggested I check the servo by removing the pipe that comes in from the inlet manifold and see if I could feel any brake fluid in the servo. Well I did this and there was brake fluid in the servo.

I have started looking around for a new servo and this is the reason for this post. I have read some of the previous posts about this topic. My understanding is the one in my car might be a Girling Powerstop? Also that when these were no longer available these units were replaced by Lockheed units?

Anyway my question is, what would be a good replacement for my servo. When looking on ebay there seems to be different ratios that seem to range from 3:1 -1:9? I am assuming this is the added force that is applied to the braking effort of the pedal.

Brake Servo.jpg

I did find a tag on my servo that had the following info on:

257 792 7
910-C34

On the other side it has the number 92.

Any advice much appreciated.

Thanks
Dan

[Mike]

Re: New brake servo

Hi Dan

Happy New Year

Sorry to hear of your servo problems - whilst I can't help on what your replacement should be, I may be able to lighten your uncertainty, and make your new one easier to source.

Servos operate by allowing atmospheric pressure to generate a fixed pressure to a diaphragm, so they provide a fixed and consistent additional pressure to the brake master cylinder. Therefore it is not correct to state that they have a ratio of boost: as you press harder on your brake pedal the ratio of assistance actually reduces, as the servo provides a consistent pressure, whilst the pedal pressure varies.

Therefore when looking for a new servo, the main objectives are:
To find one with an equal diameter diaphragm inside (the external diameter of a single diaphragm servo is a pretty good indicator of what is inside)
One which mounts via the same fixing holes (or very similar) to the bulkhead
One which will accept a master cylinder with the same diameter bore, and the same outlets to allow simple connection to your existing brake pipes to make it easy to swap.

A group of us went through a good learning curve with the Cabrio and Sportster servos - Marlin recommend using the standard Sierra servo for the Ford engined Cabrios, but as this was too big to fit alongside Rover V8 and BMW engines they recommended a Metro servo. We all went down this route, but we all knew it was inadequate - dangerously so: they were insufficient to stop our cars safely.

To cut a long story short, we found there are Dual diaphragm servos available which effectively increase the surface area of the diaphragm, by having two, which increase the additional pressure applied to the master cylinder within a given diameter servo unit. We found a 7" Dual diaphragm provided similar boost to the standard Sierra single diaphragm 9" servo: my point being, don't get too hung up looking for an exact replacement of your old (and expensive?) servo when there are modern low cost equivalents available, particularly if, as in your case, you will be buying a new master cylinder to replace your leaking unit.

[b_caswell]

Re: New brake servo

Hi Dan,
Fortunately your servo is the remote type as used on small Triumphs. Quite few Triumph part suppliers do a replacement kit. Canley Classics or Rimmers stock these.

Its not a direct replacement you do need to buy the whole kit.

[stevejgreen]

Re: New brake servo

Not that it makes too much difference, but as you have a Mk1 car, so Triumph derived, do you know what your donor car was? Not many early Triumphs were fitted with servos!

Brake fluid in a servo suggests that the master cylinder seals have failed. If the brake fluid level has dropped that much it sounds catastrophic, but recoverable. The internal parts of a servo should be resistant to brake fluid but it would be best to have it cleaned and refurbished.

I have heard good things recently about J & L Spares for refurbished parts. They might be able to help, but Ben might be along later with Triumph specific advice. (Beaten whilst typing)

[b_caswell]

Re: New brake servo

The only way the single line master cylinder would leak is past the piston and down the pedal. If there is fluid in the remote servo diaphragm its the seals in the servo leaking. Often this produces smoke from the exhaust as the fluid gets into to vacuum side of the servo.

[cameronfurnival]

Re: New brake servo

a popular choice would have been the mgb servo as they are remote items and were often added after-market. this sort of thing...
$(KGrHqR,!rgF!6n5wJSTBQVc(ogJg!~~60_57.JPG

[stevejgreen]

Re: New brake servo

The question then is, is it brake fluid, that's in the servo? Or contamination from the vacuum system.

[ggrum]

Re: New brake servo

Looks identical to the AP Lockheed unit for single line braking systems I fitted a few years ago. Can't recall the boost ratio but I think it may be dependent on the diameter. At the time (mid 90's) they came as Types 6, 7, 8 & 6 7/8 which may relate to the diameter. I can't remember who the supplier was but once you have found one I guess you could ask them which type is the same diameter as yours.

[cameronfurnival]

Re: New brake servo

My guess from what has been said, and for what its worth, is that the servo diaphragm has a tiny hole in it. It will be brake fluid in the servo housing but some will almost certainly be being burn off as it sucks into the manifold - giving white smoke. Servo repair kits are available but complete new ones are available for under £200.

[scott h]

Re: New brake servo

Car Builder Solutions do a range of remote servos and master cylinders.

[Marlindan1965]

Re: New brake servo

Thanks as always for all the helpful advice and input. I have ordered a unit from Rimmer Bros which I am hoping will be straight forward to fit.

[Marlindan1965]

Re: New brake servo

So I have taken the servo off the car and I am waiting for the new part.

I have a couple of questions. This diagram is from Rimmer Bros website.

Brake Servo diagram.jpg

The way my servo is connected at the moment there isn't really a low point in the vacuum pipe between the inlet manifold and the servo. The instructions suggest there needs to be a low point in the pipe before it goes in to the servo. The diagram also shows a non return valve but says that it might not be needed. I didn't have one of these either.

So do I need to get a longer piece of pipe and make a U trap? Do I need a non-return valve?

As always and advice appreciated.

Thanks

Dan

[greyV8pete]

Re: New brake servo

So I have taken the servo off the car and I am waiting for the new part.

.............................................

So do I need to get a longer piece of pipe and make a U trap? Do I need a non-return valve?

As always and advice appreciated.

Thanks

Dan

From your existing setup I would guess that it’s fuel that has rotted the servo diaphragm.When I was fitting a remote servo to my classic I was advised to fit a U-tube trap as the old carbs tend to be a bit free and easy with dispensing fuel! I had a metal U-tube made up from 8mm bore bundy tubing (but copper will do) as the heat from the engine will help to evaporate any raw fuel that collects. My U-tube is aprox 10” overall height and attached to the inner wing with a couple of P-clips.

Some servos incorporate a non-return valve in the connector, but if not then you need the one that fits in the vacuum pipe. Peter.