Lotus Elan

Soft brake pedal: master cylinder choice 0.625/0.7/0.75 ?

PostPost by: huggy » Sun Jan 13, 2019 1:27 pm

Another bore question...sorry :wink:

My 64 DHC S2 had its servo removed in the past.
Brake efficiency is OK but I feel brake pedal too soft and with too much travel (std Elan caliper not+2 ones).

I have ckecked leaks, bleeding, pads: Everything OK.....I just found freeplay in pushrod: corrected but not a big improvement :|

I would like to know:
- If Elan fitted with or without a servo used the same MC ? which one ?
- Confirmation that going from to a smaller bore MC will make travel shorter but will need more pressure: 0.75 to 0.625 = harder pedal but less travel ?

Thanks !! :D
IF IN DOUBT, FLAT OUT !
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PostPost by: nmauduit » Sun Jan 13, 2019 2:11 pm

Bonsoir,

I thought brake boosters (servo) were only offered starting 1966 (SE package on S2s)... you may want to check the actual cast number on your master cylinder to make sure of its exact specifications (bore diameter).
http://gglotus.org/ggpart/elanxref.htm#brakes
gives some numbers to get started, more details are on the net
lotus-suspension-f42/tandem-master-cylinder-t19064.html
...

The hydraulic leverage is the ratio of slave area by master area, so for a given slave the smaller the master cylinder diameter, the higher the leverage (that is less effort, more travel). So going to .75 to .625 is going to produce more travel and less effort (by 44% : .75^2/(.625^2) ).

It is often admitted that 2 seaters elan do not need a servo brake (I have a 11/16" bore MC on my street S4se, tandem but close to the stock mount on non servo S2 Girling 64068325, and when all is well find it just fine even with 16P calipers - I understand .75 were Plus2 MCs).

Soft pedal is quite often a case of air in the system, at times an issue of pads being knocked back for a variety of reasons. I would investigate that route before anything else (and it can be less obvious as it seems).

good luck !
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PostPost by: huggy » Mon Jan 14, 2019 6:32 pm

Many thanks for your detailled answer.
Much appreciated !
IF IN DOUBT, FLAT OUT !
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PostPost by: jeff jackson » Wed Jan 16, 2019 10:46 am

Have you checked the brake pipes?
My Plus 2 was similar to what you describe. Turned out that the pipes being old were bulging a bit under pressure. Not that it was an MOT fail or in fact unsafe, but just a soft pedal feel.
Changed them for Hel Performance brake pipes and now brakes are as they should be!
Regards
Jeff 72+2
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