[Mgs] Brake failure - What caused it?

Max Heim max_heim at sbcglobal.net
Wed Aug 25 22:37:45 MDT 2010


Hmm, I wouldn't expect to see a residual pressure valve on the disc brake
half of the MC. My American car only has one on the rear line for the drums.

My MG has only a single MC, so I would have thought the RPV would be after
the split.

--

Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires



on 8/25/10 8:34 PM, Charley & Peggy Robinson at ccrobins at ktc.com wrote:

> Think it through and keep in mind that the rear piston pushes the front
> one when there is pressure in the master.   Also, there is a trap valve
> on each outlet.
> 
>     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.  The trap
> valves give us that little bit of drag we're used to seeing.  Without
> the TVs/RPVs we'd have to move a lot more fluid with each brake
> application than we do with them, plus they compensate for wear to a
> limited degree.
> 
>    My take on my intermittent problem is that the rear trap valve
> sometimes sticks open when the brakes are released.  This allows the
> pistons to retract all the way;  meaning that the next time the brake
> pedal is pushed there isn't enough fluid in that cylinder to engage the
> wheel cylinders before the pedal runs out of travel.  Since there's no
> pressure in the rear cylinder the front piston doesn't get actuated
> either, so pedal to the metal.  When the pedal is pushed a couple more
> times and the trap valve resumes working; say it comes unstuck or a
> little piece of debris gets flushed out,  normal fluid distribution
> occurrs and  the brakes work OK.  Until the next time........
> 
> A few years ago when this problem first surfaced I replaced ther master
> cylinder with a rebuilt unit.  However it was a bare bones unit.  I had
> to use the valves, reservoir, etc., from the old one.  Things were fine
> for a while but now the problem is back.  I have a new reproduction
> master coming from LB Car Co (Moss 180-176).  Hopefully the valves are
> new too.  Meanwhile the car is sidelined.
> 
>    CR
> 
> 
> 
> 
> 
> 
> 
> On 8/25/2010 3:03 AM, Paul Hunt wrote:
>> Explain how the trap valve, situated in the outlet port of the master,
>> can cause the pedal to go to the floor?  Where does the fluid
>> displaced by the master piston go?  And why did both circuits
>> apparently do it at exactly the same time, then stop doing it at
>> exactly the same time?
>> 
>> I say again, the whole purpose of the dual-circuit master is that if
>> one circuit fails, the other will still operate to give some braking.
>> Most of it if it is the rear circuit that has failed, very little of
>> it if it is the front circuit.
>> 
>> PaulH.
>> 
>> ----- Original Message -----
>>> Ok gang, here's the final hint:  Take a look at the trap valves.  :-P


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