Servo valve spring?


Well-Known Member
Got a new valve part for the booster, dismantled old one, and forgot where the spring goes.....blackout. There is a rubber diaphragm on the white plastic body, and there is a spring - softish, ~1" dia, free length 1.5". Which side of the diaphragm does it go please? Booster side or whate plastic side??
Spring goes under the clip on cap of the ‘flying saucer’ white housing. It applies a small amount of pressure to help the valve seal.
Its too big in dia, and length to go in there - that spring is barely 1/2" long and in dia. There is no spring this shown in either the official manual or Haynes. I can only envisage this big spring going in the white plastic part, behind the diaphragm, to help push the air valve shut...Trying to get pic off the @#$%&^ iphone. The diaphragms are slightly different where the white plastic rod from the slave cylinder is attached - the flexible lip is different - on the old one the spring fits neatly around the outside of the central plastic part, but on the new diaphragm there is rubber there.
Old valve body on right with new diaphragm; spring on old diaphragm. EDIT - the actual air valve on the new plastic body is stiffer than that on the old body - maybe I am supposed to toss the old big spring?

Geoff at Wins says he has sold 150 of these without comments coming back. I guess all I can do is check that it all works OK...unless somebody here has any info to offer? FWIW the Oz made PBR VH44 servo shows a decent size spring in the valve housing, outboard of the diaphragm.
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With the spring fitted outboard of the diaphragm all seems in order, with the old diaphragm. Only possible test left is to reassemble without the spring....but why bother if its working? Found a diagram of the Delphi rebuild kit (LK1165) which does not show a spring in there, so mine seems to be an anomaly. Getting an exchange unit from UK is uneconomic if I have to ship the old unit back.
Mystery sort of solved. Got a late model booster - supposedly off last P6b imported to Oz, around 76, 77. It has a big dia soft spring in the air valve housing, slightly different from mine - a little conical . Surprising that nobody has seen these before.
The spring acts on the diaphragm in the air valve assy to push the air valve piston back in it's bore, without it the brakes may hang on.
Yes, we know that now, BUT why is it not shown in any of the diagrams, and NOBODY has reported seeing one until now? Note that I have reported the late air valve is noticeably stiffer than the original , which has the spring.
If the air valve piston is fitted with two seals there is an unofficial mod to remove one of the seals to prevent brake hang on.
Re this old booster I bought - it had around 1/2 pint of nasty looking fluid in the chamber - obviously going out for rebuild.
Also , it had a short pipe on the output to connect to the T piece. Something to learn from one end - when you overtighten brake union tube nuts behind a flared end , the end of the nut starts to expand over the flare, and this makes them very hard to pull out of whatever they are screwed into, and the expanded part needs to be filed off before they will go back in. I know that having a male thread they are not strictly 'nuts', but thats what I have always called them - did a year in a brake workshop in another life. So, be careful tightening these 'nuts', and check their condition before refitting.
sdibbers said "If the air valve piston is fitted with two seals there is an unofficial mod to remove one of the seals to prevent brake hang on."

Yes, useful, but WHICH ONE? Or doesnt it matter?
It doesn't matter because pressurised brake fluid gets between the two seals and holds it in place and doesn't let it function properly.
The big spring within the air control valve was never part of the original specification, hence why it is not shown within the workshop manuals. It is known as a modification spring used as stated above to prevent the brakes from staying on under light brake applications. The air control piston can be slow to allow the brakes to release. This spring is meant to prevent that annoying behaviour. I have used them for at least 30 years with great success.

Ron, I was confused initially, but finding that spring in another booster of the period reassured me. I never found one in a diagram of a Lockheed unit, but PBR documentation shows such a spring in their models, so now convinced.