[Mgs] Brake failure - What caused it?

[Charley & Peggy Robinson]

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