why does my mcpv only work first time?

Hello from Bonny Scotland.

I have one of your 1956 f100 ford trucks.

I had the cpp firewall mounted brake system, with the mcpv upgrade.

For the last month, it has been fine.

Yesterday, it only worked once or twice before I lost all power to the brakes -pedal hard and I nearly took out a few cars...

When I switch the engine off then back on, it works again.

Vacuum hose looks fine.

Would a failure to properly bleed the servo/cylinder at installation be a reason?

I can't understand why it was working for a month, but has now stopped.
Could this be caused by a valve in the mcpv? Is there something I. Should "reset"?

How best to test to isolate the faulty sysytem/component?


Any ideas?

Cheers,
QB