@alia176 I hear you about permanently heavy vehicles and yes if your valve is fully open then maximum braking effort is being applied to the rear brakes.
The first problem I see is about balance, it is a fact that the front will dip during heavy braking and the rear lift, but it is about the amount of dip/lift dictated by not only the braking effort but also the surface you are braking on. A fixed valve cannot cope with the variables involved for example a wet surface, you brake hard to prevent an accident on the motorway, there may not be enough traction to change the vehicles body/axle position,, the LSPV keeps pressure reduced to the rear brakes, but in a fixed valve set at fully open you still have full braking effort to the rear wheels regardless of whether loaded or not, the result is the rears may lock spinning the vehicle.
Another point is that modern vehicle brakes tend to be excellent and so it is natural to get out of a compact with great brakes and get in an 80 with a braking system that was designed over 30 years ago, add in you are also stopping a vehicle that weighs a lot more than the compact you just got out of, so comparisons are unavoidable. Being a mechanic I might drive a dozen different vehicles every week, and there seems to be very little difference in the braking characteristics between them and this is not a coincidence, you may find the exact braking system brand on a range of different marques.
In my opinion if the 80 brakes have a flaw it is in heat dissipation, I can drive my diesel 80 quick enough around the local area with no passengers and still get brake fade, all four brakes hot and smelling. I fitted dimpled and grooved discs, all new calipers and pipework, did it help? If I am brutally truthful I spent over 2000 GBP and reckon I gained a small improvement, all I really got out of it was peace of mind that the whole system was in good shape. If 'spirited' driving is in your nature then in my opinion is physically bigger brakes to help with the heat are needed and not removing components that have been tried and trusted over many years.
A final note, fitting bigger wheels and tyres to an 80 seems to be a 'right of passage', and is clearly the norm for many if not most of the forum members, but this simple bolt on change will increase the braking distance for a given pedal pressure, then along come the complaints that the brakes are no good.
Regards
Dave