PWDA valve problem

[County3.9]

I just changed the LH rear brake slave cylinder (drum brakes) on my 86 county 110. When trying to bleed the system the PDWA valve keeps isolating the rear system. (PDWA valve isolates the front or rear circuit in the event of a line failure or major leak. This way you still have some brakes. It sits on the drivers side firewall just above the chassis rail, it incorporates the fail light switch) I've done this job many times before but have never had this problem.
I have been able to reset the valve by starting the engine and pumping the brakes but when I start the bleeding process it isolates again:(

Any suggestions?

[Blknight.aus]

do an initial reverse bleed OR once the system is reset use a suction brake bleeder on the rear or a pressure bleeder on the resivior to get the initial fluid to the bottom end.

you can also remove he light switch and fit a locking pin to some types

if you need to you can come borrow my vac bleeder or if time allows on the weekend I might be able to come help out.

[County3.9]

do an initial reverse bleed OR once the system is reset use a suction brake bleeder on the rear or a pressure bleeder on the resivior to get the initial fluid to the bottom end.

How do you do a reverse bleed Dave?

[Blknight.aus]

check the first post for the edit....

but its a case of draining the MC and then using a syringe to push the fluid backwards through the brake system from the Slave to the master.

gravity bleeding works 9/10 times once you get the fluid past the Pressure differentiating safety setup bit.