Lockheed Brake Servo

[timliggins]

Back in July 2004 there was a thread about problems with brakes sticking and spongy pedal after fitting a Lockheed replacement for the original Girling servo. One reply suggested that the air valve on the servo should be pointing upwards. Since I have a similar problem and my servo is mounted with the air valve downwards, does anyone know if the orientation of the servo unit is important?

Tim 1970 S.4

[john.p.clegg]

Tim
the problem of the sticking air valve is overcome by fitting a light spring under it.as for the orientation,i think uppermost is best
John

[timliggins]

Thanks John

I have been searching around on the net and found the same topic on a Scimitar forum. The suggestion there is that the problem lies with the piston seals in the air valve. Apparently, 2 O rings are fitted to the piston as a precaution and this can prevent the piston returning to its correct position. Removing one of the rings allegedly provides a comlete cure.

Is the spring you mention available from parts suppliers?

Tim

[Foxie]

Hi All,

I recently fitted a new Delphi Lockheed Automotive servo, supplied by Classicar Automotive to my Plus Two.

Investigating the diminished brake performance after fitting, I discovered from the spec sheet supplied that the servos on the Basic Elan and the Plus 2 are different, although externally similar. The Elan servo (Part No. LR17818) has a slave cylinder dia of 11/16" and a boost of 1.9, the Plus 2 servo (Part No 18221) has a slave cyl dia of 5/8" and a boost of 3. Classicar did not seem to be too clear about this as they had sent me the wrong one. (This is being rectified at present)

The fitting instruction stipulate that the air valve be angled downwards at an angle of 30 deg from horizontal, and the main axis of the servo has the nose inclined upwards at an angle between 25 and 40 deg.

I believe this orientation is to enable complete bleeding. If the air valve was mounted on top, the piston might be running dry, as it forms an airtrap.

Sean Murray

[rdssdi]

I have purchased two Lockheed boosters for my 1969 Elan +2. As the restoration is in progress they have yet to be installed. I have part number LR10117. This is the booster with an "installation" kit. Are these boosters proper for the +2?
Bob
1969 Elan +2 (Federal, LHD)

[Foxie]

Bob,

The Plus 2 installation kit is LE10117, which includes servo No. LR18221.
original Girling No. 64049354.

The Elan installation kit is LE72696, which includes servo No. LR17818, original
Girling No. 64049178

Although I have no details of the twin servo set-up, it seems you do have the Plus 2 type.

Sean Murray

[Foxie]

Hi Bob,

Glad to be of help, find it hard to manage messages on the forum. If you are considering modifying the fibreglass to fit double servos, would you not consider modifying the plumbing instead to use a single one ? All the non-Federal Elans seem to manage fine on one, it won't change the braking, just (remotely ) the safety margin

Sean

[timliggins]

I've just carried out the mod that was mentioned on the Scimitar forum, i.e. removed one of the 'O' rings from the air valve piston and, hey presto, a perfectly working servo! This mod was recommended to our Scimitar friend by Lockheed so is safe.

Tim

[type26owner]

Tim,
Long been known that having dual bi-directional seals on a piston causes a lockup problem potentially. If one seal leaks so the area between the seals is pressurized then the two seals commonly get loaded in opposite directions and exert a rather large clamping force which can solidly immobilize the piston. I've seen this happen with hydraulic, pnuematic and vacuum applications. Just goes to prove having an idea does NOT mean it's a good one necessarily.

BTW, o-rings make very poor quality dynamic seals particularly on pistons. Quad-Rings are a vast improvement. A Quad-Ring will not fail due to spiral twisting. An o-ring commonly fails from just one section of the o-ring rolling along in it's groove while the piston is moving and the rest of the o-ring stays put. Best practice for an o-ring is to use it as a static seal only.

Just guessing this info applies in this case. If there a vent between the o-rings then of course it would not.
-Keith

[timliggins]

Keith

Ooooh, far to technical for me! The ring I took off was square in section. Is that a 'quad ring'? I thought they were called 'O' rings because they were round (like a Polo mint)