This is the 3rd complaint from a Auto mechanical workshop when trying to bleed a conversion Master Cylinder/ Booster. (VT/VX/VY into VB to VS)
I can't bleed the back brakes (even bench bleed) your master cylinder is no good. (Mechanic with 40 years in the trade.)
What happens is through a bubble or over energetic pumping of the brake pedal, the front brakes prime before the back brakes and the resultant pressure differential activates the Brake fail switch - more correctly the brake fail piston is moved by the uneven pressure on either side and moves away from the high pressure which activates the brake fail switch. What also happens is the brake fail piston moves across and also blocks off the exit port so little to no more brake fluid will be lost. In a split front/back brake system this is a fail safe of sorts, at least the front or the rear will still work. The brake system thinks that there is a failure of brake line or slave cylinder that is leaking fluid, blocks off the "low" pressure line and seals it off against more fluid loss.
I don't know when split systems became common in Aust but at least VB commodores had them(1979)
Can you reposition the brake fail piston to the neutral position? short answer YES.
If you are lucky remove the brake fail switch and work the piston back to neutral position with a scriber.
If you are unlucky disassemble and reposition the piston.
You can hydraulic the brake fail piston back into position but that's a lot more complicated.
Lord help these mechanics if they ever get a brake fail switch activated, probably the customer will pay for a new master cylinder.
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