Lotus Elan

Brake servo non-return valve

PostPost by: vernon.taylor » Thu Nov 07, 2013 11:35 am

Salut

Your indulgence and knowledge is humbly requested:

- a mate is having trouble with his servo-assisted brakes. Being I thought clever, I told him it was because he didn't have a vacuum non-return valve between the manifold and servo. He told me there is one at the entrance of the servo (a plastic bit with a grommet - like all servos I've seen) - we took it out and tested it and he's right. So what is the in-line one for ? Safety ?

Merci.

Vernon
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PostPost by: billwill » Thu Nov 07, 2013 1:31 pm

The inline non-return valve on the carburettor side, ensures that the vaccum reservoir, built into the front chassis crossmember, retains its vacuum, without a reservoir the action of the brake servo would vary dramatically depending on what the engine was doing at the time. It also ensures that the headlights don't wobble up & down depending on what the engine is doing.

I have a strong suspicion that the failure of that valve on my car (many years ago) whas what caused me to have two shunts into the back of a vehicle in front of me and made me distrust servos so I removed mine and have run without assisted brakes ever since.

The standard one on an elan is fitted on the fat rigid pipe near the steering rack, but you can easily fit one near the carburettor. Thee are some small suitable ones sold for modern fuel systems which look good for this purpose.

Image

http://www.burtonpower.com/catalogsearc ... turn+valve

You may also have one in the T-piece connector to the vacuum reservoir.
Image
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PostPost by: Chancer » Thu Nov 07, 2013 2:28 pm

vernon.taylor wrote:Salut

Your indulgence and knowledge is humbly requested:

- a mate is having trouble with his servo-assisted brakes. Being I thought clever, I told him it was because he didn't have a vacuum non-return valve between the manifold and servo. He told me there is one at the entrance of the servo (a plastic bit with a grommet - like all servos I've seen) - we took it out and tested it and he's right. So what is the in-line one for ? Safety ?

Merci.

Vernon


Salut Vernon, would that be the person posting about his brake servo with control loading spring issues?

If so I wondered whether the pulsations that he is experiencing may be down to a defective or missing valve or I have a strong feeling that the vacuum line to the servo should have a restrictor to prevent it.
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PostPost by: Hawksfield » Thu Nov 07, 2013 5:17 pm

HI

Your getting mixed up between the brake vacuum take off and the headlight vacuum take off.

The brake is at the bulkhead and the Headlights at the radiator end of the manifold, two complete separate systems.
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John

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PostPost by: billwill » Thu Nov 07, 2013 10:04 pm

Hawksfield wrote:HI

Your getting mixed up between the brake vacuum take off and the headlight vacuum take off.

The brake is at the bulkhead and the Headlights at the radiator end of the manifold, two complete separate systems.


Aye, sorry, that is on the +2.

What I described is on the 2-seater Elan.
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PostPost by: vernon.taylor » Fri Nov 08, 2013 8:29 am

Salut les gars

D?sol? - I should have mentioned it concerns a +2 - nobody here on the forum.

The headlight takeoff has a seperate non-return valve (that works). The servo takeoff usually (I believe) has a non-return valve in the pipework near the bulkhead. My question is 'what is the function of this non-return valve' given that all the servos I've seen (about five ;-) have a non-return valve at their input. Is it a safety measure in case the servo non-return valve fails ?

Merci

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PostPost by: rgh0 » Fri Nov 08, 2013 8:38 am

Hi Vernon
My 73 Plus 2S 130 /5 only has the non return valve in the servo unit, it does not have one at the bulkhead intake connection for the servo like there is for the headlights vacuum connection at the front . I believe this is the standard arrangement.

cheers
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PostPost by: vernon.taylor » Fri Nov 08, 2013 11:32 am

Salut Rohan

Not my car but this is what I am talking about. I've seen it on a lot of cars:

non-return valve.jpg and


@+

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PostPost by: billwill » Fri Nov 08, 2013 12:52 pm

Hmm, so no vacuum reservoir on the brakes of a plus 2. ?

In which case I expect Lotus put that one in, just in case a servo was used that had no non return valve.

It won't matter that there are two and it will be slightly safer.

~~~~

The valve in that picture looks identical to the one I took off my two-seater Elan. It had completely failed. I can see daylight through it. But WHEN it failed I do not know since I ran for nearly 30 years with the servo still mounted in the car, but not connected at all.

I finally got around to taking it out 3 years ago and sold the servo on eBay.
Last edited by billwill on Fri Nov 08, 2013 4:31 pm, edited 1 time in total.
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PostPost by: billwill » Fri Nov 08, 2013 1:00 pm

On further thoughts, in my opinion.

The volume of the vacuum pipe from rear carb to servo is not big enough to significantly act as a reservoir for the brakes and so the non return valve would have no significant effect on braking, but perhaps it has enough volume to affect the fuel mixture. Without a valve, air would tend to rush back and forwards through the side hole of number 4 carb, causing erratic air mixture.

So putting a non return valve near the carburettor may prevent some odd misfiring situations.
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PostPost by: Hawksfield » Fri Nov 08, 2013 1:32 pm

HI billwill

You are probably correct for the reason to have the NR valve close to the manifold and in addition a length of tube was used. My car was originally installed like this but had to be changed because of a faulty NR in the tube was failing, I now have an inline NR in the rubber connecting pipe, as my servo does not have a NR in the inlet to the servo can.
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