Headlamp vacuum circuit

[RedS4]

I don't know if it's "standard" or not but my servo has always been fed from a brass "T" piece in the front of the inlet manifold, the other branch of the "T" feeding the headlights. The two non return valves are screwed directly into the "T"

I have a similar arrangement with two brass fittings on the end of the manifold, with the two large valves (mentioned previously) slightly down-stream.

[billwill]

Nah, I think you get the same effect even if there is a non-return valve on the server (as there must be) because what you actually do is stamp the brake in the emergency, realise you have stalled the engine, release the foot brake then realise you are still roling so you re-apply it & find no assistance. Because the 'vacuum' available has all has been sort of used up.

Try it on a track or quiet road sometime. Hit the brake hard without hitting the clutch.


Logically it does not make sense If you have managed to stall the engine, then theoretically the car must have stopped, so why does it still roll forward again. Possibly it is due to energy stored in the doughnuts. It's hard to believe & harder to reproduce, but I honestly believe it happened to me twice. The first time the nose was wrecked against the car/van in front (and replaced with one with S4 wheel arches) The second time I only bust a headlight pod, because the car in front was one of those lorry things without a proper bumper, just a chassis beam at my pod height.

Anyway that's my story & I'm sticking to it

[oldelanman]

Nah, I think you get the same effect even if there is a non-return valve on the server (as there must be) because what you actually do is stamp the brake in the emergency, realise you have stalled the engine, release the foot brake then realise you are still roling so you re-apply it & find no assistance. Because the 'vacuum' available has all has been sort of used up.

At the risk of labouring this slightly off-topic point....No offence Bill but I still think that your unfortunate experience was down to a faulty servo or vacuum leak and was not typical of the way a servo works. I would not want people to discard their servos for this reason - although there may well be other reasons for doing so!

If the servo is functioning correctly, has no leaks, is protected by a non return valve (wherever it is placed in the system) and is fully "charged" with vacuum I cannot agree that all of this vacuum can be used up in one brake application, no matter how hard you stamp on the pedal. At rest the diaphragm is pushed fully back by the large internal spring to the rear of the can. When the brakes are applied air is admitted behind this diaphragm at atmospheric pressure to produce the assistance force and the piston and diaphragm will move forward in the can pushing on the secondary or output hydraulic piston. Provided there is no air in the hydraulic system, this movement should be small - just enough to take up the clearances between pushrod and piston and the pads to disks before hydraulic line pressure can be generated and thus the volume of air admitted is also small compared with the total volume of the servo can. When the brakes are released the two sides of the diaphragm are connected together again, the spring pushes back the diaphragm and the pressure on either side of it will equalize, so a small volume of air at atmospheric pressure now occupies the total volume of the can and it's pressure will drop as a result (P1 x V1)/T1 = (P2 x V2)/T2 etc. This will not "use up" all the vacuum, the pressure in the servo will be increased by the injection of air but will still be significantly lower than the atmospheric pressure outside so the servo will still function on the next and subsequent brake applications - albeit at a somewhat reduced efficiency.
Sorry that explanation is a bit wordy but I hope you can see my reasoning.

Try it on a track or quiet road sometime.

For 21 years I worked at Ford's Boreham Proving Ground (also home to the Ford Competition Department) where I was responsible amongst other things for commercial vehicle brake testing.

I rest my case

[billwill]

None the less, I believe it happened to me twice and as far as I am aware the Non-Return valve was in place. If it was in the fat metal pipe by the steering rack it may still be in the car, I'll check some time.

There's two issues actually
1. Is it possible to stall the engine by hard braking and yet still have lots of forward momentum (enough to smash the nose),
2. is there sufficient vacuum in the feed pipe and chamber of a Girling servo to make two successive strong applications of the brake?

[oldelanman]

1. Is it possible to stall the engine by hard braking and yet still have lots of forward momentum (enough to smash the nose)

Yes. Locking the rear wheels stalls the engine but doesn't stop the vehicle moving forward.

2. is there sufficient vacuum in the feed pipe and chamber of a Girling servo to make two successive strong applications of the brake?

I believe there is. Although as I said before you will have to push a bit harder on the second application.

Best regards,

[dlbarnes1]

bill308 asked " Anybody find an inexpensive one way valve for in-line fitting? "

I have found a suitable valve, a Swagelok #B-2C4-1. This is an inline check valve, brass body, 1/8" FNPT ports, for pneumatic service, vacuum to 125 psig.
Can't remember the price, probably around $20.
Google Swagelok for a local source.

Dave - 72 Sprint DHC