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Re: [Mgs] Brake failure - What caused it?

To: <ccrobins@ktc.com>
Subject: Re: [Mgs] Brake failure - What caused it?
From: "Paul Hunt" <paul.hunt1@blueyonder.co.uk>
Date: Thu, 26 Aug 2010 09:41:03 +0100
This completely misses the point of dual circuit brakes, which is that even 
if one circuit is completely open, the other will still work, albeit with a 
slightly longer pedal.  Also single-circuit masters don't have these trap 
valves, and neither as far as I can see do the boosted dual masters.  Rear 
slave cylinders do not fully retract anyway if you have the shoe and 
handbrake cable correctly adjusted, and just how far are discs going to push 
caliper pistons back unless you have massive hub wobble?  Incorrectly 
adjusted rear brakes and handbrake do indeed cause a long pedal, but whether 
they could ever be maladjusted enough to cause the pedal to go to the floor 
is a moot point.  But even if they could, that could still only happen on a 
*single-circuit system*, not a dual circuit.  A trap-valve in the rear 
circuit may well 'conceal' a maladjusted handbrake from the footbrake pedal, 
but not from the handbrake lever itself, which would probably come all the 
way up without applying the brakes.  If you handbrake is working correctly, 
then the trap valves aren't doing anything anyway.

You have two major questions to answer - where did all the fluid go from the 
front circuit?   And what is causing *both* circuits to fail?  Until you 
find and fix at least the second, it would be highly dangerous to drive the 
car anywhere, let alone on a public road.

PaulH.

----- Original Message ----- 
>    For those who haven't read up on  Trap Valves (TV) - called Residual
> Pressure Valves (RPV)  in US parlance - they are one way valves that let
> high pressure fluid flow through from master to slave but shut off low
> pressure reverse flow from slave to master.  The idea is to keep the
> return springs at the wheel cylinders from retracting the slave cylinder
> pistons all the way into the cylinders.  Likewise to keep the caliper
> pistons from being knocked all the way back in by the rotors.
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