just use the vs master cylinder they work fine some are one inch any but the rest are 15/16
a brake mechanic i know has the twin pistons on his vn it has a 15/16 master cylinder he said the pedal moved slightly but the more pedal movement and smaller the master cylinder the more torque you can exsert
No, use the VT Master. VB to VS Commodores had a choice of 15/16inch or 1inch bore master cylinders. This is not suitable for the VT twin spot brake calipers. The VT master cylinder is a 1inch bore master cylinder, but is a quick fill. This means that the first section of the master cylinder is actually larger (1 3/1 6inch bore). The quick fill design displaces more brake fluid on the initial application, to help move all 4 (front) pistons and maintain a good pedal feel with minimal travel. Without a VT master cylinder, pedal travel will be increased. In other words, your pedal will go further to the floor before the car stops (some ABS vehicles do not suffer from this problem, but only some!!!).
Therefore running a VS Master/Booster combo which were designed for much smaller brakes/master cylinder will basically eliminate the benefits of the twin piston caliper/greater braking surface.
The VS Booster being single diaphragm doesn't have enough power/push/vaccum to be able to give you the solid pedal feel, you need to push the pedal harder, through the threshold to be able to build up the power to push the volume of brake fluid required to push the pistons out, through the lines.
If your brake mechanic said that, and said that a brake pedal with a 10 - 40% no pedal threshold was fine then he's a bit of a loony. With 40% of your pedal gone - just 'free pushing' what is the point? You might as well have the weaker VS single piston brakes in and just use more force pushing down, at least you have a more responsive pedal where you can actually feel the brakes. Rather than pushing through a 'free push threshold' and either not engaging the brakes at all or locking them up.
As I said, pairing the VT brakes with a VS Booster/Master or even a VT master with a VS Booster is a disgusting combination. It does the larger brakes no justice and IMHO it is a waste of money without the larger more compatible VT Booster/Master.
a long and spongy pedal would probaly be the resualt of incorrect bleeding
The system was bled and re-bled 5 times and re-set up twice. The VS Booster and M/C aren't compatible with VT calipers to the extent of getting a full pedal and responsive prominent progressive braking.
