CosKev3
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| posted on 1/7/14 at 09:44 PM |
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Brake master cylinder expert's help req please
http://www.geocities.ws/julianvz/Sierra/Repair/br_sys_m_cyl.html
anyone know if it's ok to use a Sierra master cylinder with the stop pin removed?
And what is its purpose?
Its in picture 13.6 in the link above.
Reason I ask is due to me not being able to bleed my front brakes initially,there was no fluid coming through the front two ports of mc at all,could
not draw any fluid through using vacuum,but rears bled up fine.
so I stripped mc down, and found nothing wrong inside!
it was at this point I wondered if the stop screw might have been put in when the mc internals were in the wrong place,restricting the front part of
mc internals.
so I left the stop screw out and got fluid through to the front brakes easily by vacuum, then bled them up fine.
the brakes now feel spot on, mc is working as it should with no dodgy feel/noises etc.
so just need to know for peace of mind its ok to leave it out!
Cheers
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benchmark51
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| posted on 2/7/14 at 06:44 AM |
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Not an expert, but I would refit the stop pin. The pistons should be depressed when fitting the stop pin, thus restricting the rearward movement of
the secondary piston. Could be this wasn't done during original or last assembly. It wouldn't be there if it wasn't neccesary.
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Dick Axtell
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| posted on 2/7/14 at 08:12 AM |
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Stop Pin!
Hi. Sierra mcyl stop pin is essential, for correct operational assembly of this mcyl. Secondary piston return spring has to have sufficient fitted
load to return both pistons. Primary piston return spring is assembled in trapped length arrangement, to ensure that secondary piston + spring
are moved forwards, together with primary piston assembly. The aim is to achieve simultaneous cut-off, (and consequent pressure generation), in the 2
chambers. To enable fitment of the primary piston assy, you have to retain the secondary piston assy in its design position. Otherwise repeated
sliding of the critical seals over the cut-off ports couls cause damage to the seals edges.
If there was no fluid output from front outlet ports, but you successfully bled the rear brakes, then it sounds as though your suggestion, that the
stop pin had been fitted incorrectly, was right.
[Edited on 2/7/14 by Dick Axtell]
Work-in-Progress: Changed to Zetec + T9. Still trying!!
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CosKev3
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| posted on 2/7/14 at 09:46 AM |
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Ok thanks alot for the info 
Next question is, where in the piston assembly does the stop pin locate into?
When I stripped the primary piston out and looked down inside the mc I could not see the stop pin, just the end of the secondary piston assembly.
From what I understand from the above the stop pin should be on the primary piston side of the secondary piston assembly?(to limit return travel on
secondary piston assembly)
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tims31
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| posted on 2/7/14 at 11:38 AM |
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quote:
Originally posted by CosKev3
Ok thanks alot for the info
Next question is, where in the piston assembly does the stop pin locate into?
When I stripped the primary piston out and looked down inside the mc I could not see the stop pin, just the end of the secondary piston assembly.
From what I understand from the above the stop pin should be on the primary piston side of the secondary piston assembly?(to limit return travel on
secondary piston assembly)
From looking at the diagram I'd say in the section marked 'D' but don't take my word for it. This would stop the movement of
the forward piston as stated above.
Build: http://www.martinsfurybuild.co.uk/
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Dick Axtell
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| posted on 2/7/14 at 12:09 PM |
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quote:
Originally posted by CosKev3
....the stop pin should be on the primary piston side of the secondary piston assembly?(to limit return travel on secondary piston
assembly)
Correct. Assuming there is no back-stop, (e.g. servo output rod, circlip, etc.), the secondary spring could push back and partially expel the primary
piston assy. Once mcyl and servo have been coupled together, the stop pin could become redundant. However, primary spring failures have been known to
occur. In this case, the stop pin would ensure that the secondary piston was retained in its operational position.
Work-in-Progress: Changed to Zetec + T9. Still trying!!
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CosKev3
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| posted on 2/7/14 at 01:42 PM |
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Thanks both
There is a circlip and washer retaining the whole assembly in the mc.
The mc was new, so thinking must have been a assembly mistake.
Struggling to see where the stop pin was located wrongly though to stop any fluid making it into the mc?
If it was just restricting the assembly being pushed in I could understand it, as it could have been in the machined groove on secondary piston,
locking it from moving either in or out.
But I could not even suck fluid through, or blow back from the bleed nipples into mc.
Unless the stop screw has nothing to do with it and its just coincidence something was blocking up the fluid feed hole into mc! And its just cleared
now as I've stripped mc.......
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Dick Axtell
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| posted on 2/7/14 at 03:12 PM |
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quote:
Originally posted by CosKev3
...And its just cleared now as I've stripped mc....
Stranger things have happened !!
Work-in-Progress: Changed to Zetec + T9. Still trying!!
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CosKev3
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| posted on 4/7/14 at 10:07 AM |
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quote:
Originally posted by Dick Axtell
Once mcyl and servo have been coupled together, the stop pin could redundant. However, primary spring failures have been known to occur. In this case,
the stop pin would ensure that the secondary piston was retained in its operational position.
So really the only risk with mine not having a stop pin now is if the primary spring fails?
And even if it does fail I would not be at risk of losing braking totally due to no loss of fluid, just the piston would be in wrong position in mc?
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