I just completed the following:
New flanged rear axles
Turner disc brake conversions, front and rear
All new brake tubes
All new flex hoses
New dual master cylinder
Eliminated the hydraulic stop light switch and installed a mechanical switch at the brake pedal. The port on the MC for the hydraulic switch is plugged and sealed.
Today I filled the system with fresh DOT 4 fluid and bled the brakes. I found two leaks in the rear circuit. I tightened the leaky fittings and bled the rear brakes again.
With the engine not running, the brakes feel firm as they are applied, and I can see the booster and master cylinder flexing upward slightly very soon after I start pressing the pedal and as resistance builds up in the pedal.
This is the problem. The car is completely off the ground on jack stands and the wheels off. When I start the engine and apply the brakes, the pedal goes almost to the floor before I feel any solid resistance in the pedal. The booster and master cylinder do not flex up slightly until the pedal is nearly to the floor. As the pedal is nearing the floor, I can hear a change in tone in the engine as if there is an additional load on the engine.
With the engine at idle and the transmission in drive, and no pressure on the brake pedal, the rear axle turns. If I apply the brakes, the rear axle does not stop turning until the pedal is nearly to the floor, which seems to indicate to me that the booster is not applying pressure to the MC until the pedal is near the floor.
I removed the check valve on the booster and confirmed it only allows air to move towards the engine. I removed the vacuum hose between the check valve and fitting on the intake manifold and verified it is clear. With the check valve installed and twisted into position, it is still a little loose on the booster. There is one gasket in place.
When I installed the MC, I had to shorten the length of the actuating pushrod. The contact point on the piston is at a more shallow depth than the original MC. I adjusted the rod so that the rod is not putting any pressure on the piston as the mounting flange seats on the face of the booster. Lengthing it by one or two flats of the nut changes how the MC seats on the booster, so I know it's adjusted right.
What am I missing? What should I check next? I'm not going to drive it until I figure this out.
It wasn't doing this before I did all this work. The front brake pads were worn out, but the car was stopping and the brake pedal resistance was better than this.
