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Brakes 'too sharp' following servo overhaul

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connort
Posts: 10
Joined: Mon Oct 24, 2016 11:41 am
Location: Chelmsford, England

Brakes 'too sharp' following servo overhaul

Post by connort »

Hello All - Calling all Lockhead 6 7/8" brake servo experts.

I overhauled the servo on my 1967 saloon some time back but the brakes were quite some time 'dormant' while various other issues like rewiring and a bum respray etc etc. all got attended to. In the servo overhaul I bought/fitted a new main piston/stainless steel pushrod assembly from J&L (Rochdale). As I can barely remember it, the pushrod length was supposedly 'preset' and, after cleaning everything, I carefully fitted all the overhaul parts and returned the servo to the car.

When my son and I finally got the car back on the road (last September) it was instantly apparent that the brakes are almost 'digital' i.e. having very little progression. We certainly can't complain of 'insufficient braking' but it would be nice to get some decently 'progressive stopping' back.

We need to fit a new brake master cylinder so, removing the servo for attention at that point, seems an obvious move. I'm thinking that the pushrod needs adjusting (shortening) for more 'clearance' but, all knowledgeable advice on the subject, is welcomed.

Regards...
Trevor Connors

Bud
Posts: 318
Joined: Fri Feb 12, 2016 9:06 pm
Location: Rochester, NY

Re: Brakes 'too sharp' following servo overhaul

Post by Bud »

Sounds like your system is, like you said, " pre-loaded and just waiting to clamp ".
Any residual pressure valving? They may be unnecessary with servo brakes.
Check your hose connections for vacuum loss
I would then get some more play in your pedal by shortening the brake cylinder rod and check it. Hopefully your done.
Next block off the vacuum to the servo and try it with just the brake cylinder.
Reconnect the hose and zero in on the culprit.
If no change when adjusting rod throw. check M/C
If they work without the servo. then servo system needs attn.

connort
Posts: 10
Joined: Mon Oct 24, 2016 11:41 am
Location: Chelmsford, England

Re: Brakes 'too sharp' following servo overhaul

Post by connort »

Hi Bud,
Thanks for your reply on the brake servo issue (seems I can't even spell ... 'Lockheed' let alone repair the problem !!). As you advise, we'll check for vacuum leaks but I'm pretty sure all hoses/connections are OK (and there's a new valve on the vacuum tank). I'm pretty sure the answer lies in adjustment of the piston rod, which, unfortunately means removing the awkwardly sited unit.

The other possible problem area is the air valve which might (for some obscure reason) be opening too easily (i.e. at too low hydraulic pressure). Being retired I took plenty of time over the servo restoration and I'm fairly sure I didn't make any mistakes but, of course, it's always possible. Fortunately, I've got the full workshop manual for reference and if adjustment of the rod doesn't work we'll pull the unit down and recheck everything (the manual even gives one turn of the piston rod thread as 0.035" and 0.060" to 0.070" as the required clearance).

Best regards
Trevor

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