Hi Peter
Whilst I fully understand the reason for your concern - no one wants to be responsible for causing others problems of a potentially serious nature, however small the risk (I declined to supply adaptor plates made in stainless for exactly the same reason), may I offer a reasoned response for others to consider?
Like GOO, I'm not too sure that there is a real problem here, let alone one that merits action. GOO, Andrew Curtis, myself (and possibly David Siddall and Alan Hogg) all have this MC, and we all have tested our cars without any initial problems, so I do not believe there is a fundamental problem. Of course there could be an issue in the event of a valve failure, but I do not see that as a justifiable reason to ditch these MCs.
I am, therefore, quite happy to continue with my set up.
Like you, I came across this guy's web page when researching servos. On balance, I am not convinced by his comments. If you give his proposition some thought it is just not rational to assume a low pressure residual valve can cause brakes to lock up to such an extent that they will skid.
Bear in mind the original reason for all of us getting involved with this project. It was because of the inabilility to generate sufficient braking pressure at the wheels to slow our cars down in an emergency. Few, if any of us, using a Metro servo could lock the brakes up whatever the pedal pressure applied. Fitting a residual pressure return valve does not change that. The brakes can only lock up when an excess of pressure is applied. This is a function of pedal pressure, the servo boost, and the MC bore size. Pressure does not build cumulatively through repeated use of the brake pedal. Even with a failed residual pressure valve which prevents all return pressure being released, the pressure can only reach the maximum applied by the MC at any one time.
Front and rear brakes were designed by manufaturers (pre ABS) to ensure the fronts would skid before the rears. Therefore the rears should never reach a point where a skid is induced, and the residual pressure valve can do nothing to increase that pressure.
My view is that the worst that can happen is the rear brakes bind if the return valve completely fails. An equally likely failure would be for the main bore seals to fail, in which case we would not drive our cars at all.
More research is required to establish :
- That there is a residual pressure valve in this MC.
- That it was not fitted to MCs for Sierras/BMWs with rear brake discs
- If it was not originally fitted, do others support the view that it could cause a problem.
- What are the potential problems
- What is the likelyhood of any of those problems occuring
- What are the consequences of those problems if they occur.
- Until we have answers to all these we are not in a position to make an informed decision.
In the meantime, and I can only speak for myself, I am prepared to accept what I consider to be a very low level risk.
To use the H&S jargon from work, I have assessed the risks as very limited: I have assessed the consequence as very limited: therefore the decision is to take no further mitigating action, other than to seek more information, and reassess that decision in the future.
But, I do understand your concern, and I did the same as you by dipping out of supplying stainless steel adaptors when the risk was probably just as small.
The consequences, however remote, are not worth the worry when it involves someone else. If you have highlighted the issue, and others continue with this MC you have done all that could be expected of you, and they must be accountable for their own informed decision making.
Mike
Quote:
Originally Posted by peterux
I have a concern about using BMC4052 master cylinders with cars fitted with REAR disk brake callipers that I ought to raise.
The discussion above about brake pipes got me reading some more theory on Brake master cylinders and how they work. During this bit of self tution I found a reference to Brake Master cylinders designed for drum brakes have residual pressure valves built in.
According to this article....
http://www.hotrodheaven.com/tech/brakes/
the guy reckons that if you use a M/C designed for drum brakes with disk callipers it can cause them to lock up!
Now the BMC4052 was used on pre 1988 Sierra's that were fitted with drum brakes at the rear (as far as I can tell from Brakes International website).
So I'm guessing it may have a residual pressure valve built in and the Master Cylinder is not suitable for rear disk brake callipers.
This may not be the case but I have no way of telling so I thought I should bring this to peoples attention.
My apologies to anyone who has bought one of these M/C but I'd rather you didn't fit it and have your rear brakes lock up  |