I'm wondering if this is the Master Cylinder ?
It maybe just the way it's mounted / fitted that's at issue.
You should be able to do this in the garage. Check the line coming out of the master cylinder will undo (just 'crack and then close it). Get the Brakes to Lock and then quickly 'crack' the line at the master cylinder. If you get a spurt of fluid then it's a Master Cylinder Issue.
When The Master Cylinder are fitted up, it is critical that the Push Rod Clevis and the Brake Pedal are both clear of the body when at rest, ie, the Pedal should 'hang' off the Push Rod. If it does not, then at rest the ports allowing the fluid to return will always be covered over. Because if the shape and design of the seals in it, as you drive and with normal road vibration, it slowly pumps the Brakes on and the seals act as a one way valve. I can't recall if these were used in UK production, but there are shims available to pack out the Master Cylinder.
If it's not this, then The Master is likely at fault. The Seals can swell causing the same ^ and or the Residual Pressure Valve within it can act up also doing the same ^ .