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
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.
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.
> ----- Original Message -----
>> Ok gang, here's the final hint: Take a look at the trap valves. :-P
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