Originally Posted by Scarebird
no. their job is to limit pressure AFTER a certain point is reached. when you stamp on the brake hard they kick in, but during normal stops, they allow full pressure to the rears, allowing the rears to help stop the car.
Maybe I'm misunderstanding what you're trying to say, but as I read it, that is confusing. Tilton has graphs showing the output pressure curves of their proportioning valves in their installation instructions here:
The output pressure will ramp up at the same rate as input pressure up to a preset pressure (usually a very low pressure) then at a certain point the ramp rate changes and output pressure becomes proportional to the input pressure but lower (hence the term PROPORTIONING valve). The peak output pressure is also always less than the peak input pressure. I think the confusion is in your statement that "during normal stops, they allow full pressure to the rears". I think what you mean is that under light braking, the pressures are the same, but under light braking you won't be locking up the rears anyway.
On factory units this proportioning is not adjustable, but the whole purpose of aftermarket adjustable units is to allow you to change the output to input ratio. Under NO condition does the rear brake see the full input pressure that the front sees when using a proportioning valve unless you back it off to the point that it's disabled.
Proportioning valves are used in disc/disc systems all the time to account for front/rear weight distribution. The factories leave them out when possible to save cost. Pickup trucks use load-sensing proportioning valves to change the front/rear bias depending on whether the truck is full or empty.
There's nothing magic about a proportioning valve. It does not change how it operates due to the rate of pressure rise. It is essentially a pressure regulator, but rather than limiting the output pressure to a fixed PSI, it is designed to reduce the output pressure to a percentage of the input pressure. Yes, you CAN use a proportioning valve to prevent premature lockup if you have bigger rotors on the back than the front. The only issue is that you're potentially wasting money installing those bigger rear rotors if you're just going to reduce their effectiveness by dropping the line pressure.