Quite easy to diagnose..the PO has removed the proportioning valve and fitted a plain tee piece

The first time it happened I thought it was just over exuberance/inexperience (and, ahem, being out with an Integrale and a Mk1 and Mk2 Escort

I presume the solution is to fit a Minispares adjustable valve?
..check your valves out there and go easy!!
Jon