Hi guys, I have an issue where my front brakes lock far too easily, especially in the wet.
 
The car:
1990 JDM SW20 N/A Manual (non-abs) with Gen 4 3SGTE
1993+ Gen 3 Turbo brakes
Thin N/A Brake Booster
1990 N/A proportioning Valve
4 Runner 1" Master Cylinder
Tyres are Federal 595's
 
I am thinking that it might be the n/a prop valve, I have read contradicting information that the N/A and Turbo's have different brake bias or that maybe the 1" master is too big?
 
Appreciate any advice.

Firstly, I don't know what may be causing your problem, however the turbo and NA Prop valves are different in their operation. Google up 'Brake bias Knee Point' 
The M/C bore size will mainly only affect pedal pressure needed, and pedal travel.
 
Your system looks like a bit of a mixture.
As well as a later model turbo prop valve, maybe look at also updating the booster. The post '94 ones are probably the best ones to get and are a lot more powerful than the early model NA ones. You would also need a separate check valve. Your one inch master may still be OK even though 15/16" is the OEM size. 
 
 

The easiest and cheapest option might be to fit an adjustable brake bias valve.

Some feedback about my set up which is very similar to yours,
 
1991 JDM SW20 Turbo
1993+ Gen 3 Turbo brake Calipers
Stock Brake Booster
1991 Turbo proportioning Valve
4 Runner 1" Master Cylinder
Tyres are Federal 595rr's
Supra BBK front
RX8 BBK rear
 
Basically feels like the stock setup but with more bite and slllightly more pedal movement prior to initial brake contact.  Bias feels as it was with the '91 calipers.

Thanks for the response everyone. I have sourced a 91 Turbo prop valve and will give it a crack.
 
I do have one of the late model brake boosters but have not yet fitted it yet.
 
Will keep you all updated.

Do you need modify the hardlines when fitting a slim booster?