Hi, I am looking at purchasing later model "gen 3" brakes, I have read that rear 22v brakes have pistons that are 1mm bigger than the earlier model I am wondering if somebody can shead some light on the actual brake difference between earlier and later model front brakes I know they are both two piston in the case of turbo SW20's and I'm fairly sure piston size is the same?

The fronts are both twin pistons of the same size, but Gen3 have bigger discs and the calipers are mounted further out so you get more braking torque also.
Rears are both single piston, but the Gen3 is slightly larger diameter, and again the discs are larger and the calipers mounted further out.

Gen 3 also has a larger piston diameter in the master cylinder, and the booster is also different.  You should also replace these to get the most benefit.
 
Cheers
 

And to clarify. The brakes were upgraded for the rev2 92/93, along with the suspension.

The rears rotors are also thicker.

SW20 brakes:
It's not that simple to get the same braking feel as the later models if you want to update an early model car.
You will need to change calipers, rotors, master cylinder, and booster. 

Pre '92 M/C's were 7/8" bore while post '92 ones are all 15/16".
You can see the size cast into the housing under the reservoir.
 
There are least four different boosters that I know of that were fitted to Sw20's.
Some early JDM models had a 'fat shape' booster, while pre '92 ADM models had a slimline shape booster.
These both had the vacuum check valve mounted on the booster housing as part of the inlet connection.
 
All post '92 boosters are slimline and have no check valve on the booster itself, but instead have it mounted in the engine bay on the inlet manifold. So if replacing an early model booster with a later one you will need a new check valve as well, otherwise your brakes will not work properly. (dangerous actually )
 
SW20 boosters were updated with more powerful ones post 11/93 so those are the best to go for if updating.
 
 
 
 

Thanks fellas, much appreciated

I need to investigate the check valve.  The car is the early model but has had the larger discs and calipers fitted.  I recently picked up the later model MC and booster, but have yet to fit it.
 
The front and rear discs went up in diameter as well as thickness.  Another benefit is that the larger discs have double the allowable wear, this may only be on one of the ends, cant recall.
 
Cheers
 
 
 

Can I ask how much you spent on this brake conversion so far?

The proportioning valve only differs between NA and Turbo motors, not between the generations of turbos.
Both have a 60% pressure gradient to the rear, but the knee point is 30kg/cm2 vs 60kg/cm2. It makes a fair difference to the maximum braking.

artymr2
I need to investigate the check valve.  The car is the early model but has had the larger discs and calipers fitted.  I recently picked up the later model MC and booster, but have yet to fit it.
 
Cheers
 

Have you seen:
http://www.mr2australia.c./mr2play/tm.aspx?m=97679