Ferocious brakes on our M8 Greyhound

[marco]

Hello Malcolm,

See attached diagram showing the brake system on the M20/M8 armored car.
There are two hydraulic lines running from the brake master cylinder to the Hydrovac.
The extra hydraulic line actuates a valve inside the Hydrovac controlling the brake action on partial application of the brake pedal. The absence of this extra line might be the cause of the "all or nothing" attitude of the brake system or it might indicate that at some time a different Hydrovac has been installed.

Years ago we restored a M20 armored car and fitted a Halftrack Hydrovac (single line) because an original one couldn't be found.
This Hydrovac has a higher outlet pressure compared to the original M20 example.
The result was that the brake application was very similar to what you experienced.
On full brake application the car could be stopped with all 6 tyres smoking..
I have more information on Hydrovacs, i'll go through it this week to see if there is usefull information to share.

Marco