Master cylinder

[Captain flint]

Hi,
After plenty of good previous advice,I am quietly optimistic someone out there can
Help me!
I have just bought a new master cylinder to fit to my Marina disc braked Moggy.
Before I fit I believe I have to make some modification,is this right,if so what is it?
Many thanks,
Martin.

[IslipMinor]

Martin,

For disc brakes you need to remove the 'residual pressure' valve, otherwise the system that is designed to keep the drum brake cylinder seals 'open', instead will hold the pads against the discs and create a lot of heat.

It is the 'Valve cup' in the diagram below:
[frame][/frame]

[Captain flint]

Many thanks Islipminor,that's just the information I required.
Regards,
Martin

[bmcecosse]

It would have been easier/better to buy one ready modified for the job......available from JLH -and perhaps others..... I would want the job done by a reliable/trusted company - rather than fiddling with it myself. Could be difficult to explain in Court someday how you 'removed a seal from the brake master cylinder' shortly before the brakes failed.....causing mayhem....or worse......

[rayofleamington]

personally I wouldn't trust anyone to do that mod on my behalf, but each to their own. If you aren't competent to work on brakes, then dont work on brakes.
Certainly other than circlip pliers, no metal tools should be used anywhere near the m/c bore or internals.
The seals need to be carefully inspected, even if "new" and you need a VERY clean area before considering opening up the m/c.

For negating the non return valve, various methods have been used by many people including making a small hole to complete removal. If a small hole is enough to prevent disk brake pressure retention, then why do more (IMHO).

For reassembly, if you don't have brake fluid seal lube (usually white and nearly impossible to get) then don't lubricate the seals with anything other than the correct brake fluid!

Even if you dont modify, I tend to disassemble 'standard' Minor brake cylinders (wheels or m/c) and inspect before fitting! Rolled seals and swarf are not what you want to find but it's better to find (and rectify) a brake problem with parts on the bench than in a ditch.

The last "new" m/c I fitted without checking was problematic. This was a Lockheed part (previously AP, now labelled Delphi - but not really 'original spec' other than the looks) as I'd heard many bad things about the cheap copies (which were approx half the price, but deemed 'not worth the risk' by many of the parts suppliers, although still selling them).
I never managed to get a good pedal, and a month later another competent Minorist also failed to improve the pedal travel. About 5 years later the m/c was changed again and that cured the issue