My problem is that the brake pedal get soft after doing a hard corner or driving fast on uneven road surface. One pump on the pedal and it's nice and firm again. no problem driving straight down a smooth road surface.

Current brake components:
- Manual brakes with Dr. Diff 15/16" master cylinder
- OEM one piece proportion valve (see attached image)
- OEM 1970 drum brake spindle
- SSBC front calipers (see attached image)
- SSBC 11.25" inch rotor.
- In the rear I got Dr. Diff 10.7" rear disc brake kit mounted to a 8 3/4" rear end.


So far I've been bleeding with the following methods to make sure there in no air in the system: Normal bleeding with pushing the pedal with a help from a fried, gravity bleeding, bleeding with vacuum device from bleeder screw, pushing fluid from bleeder screw to the master cylinder.

I've been checking rotor run out with a dial indicator on both sides of the rotor measuring .0012" run out as the maximum.
New wheel bearings set up with OEM torque spec.

After all this I still have the same soft pedal and I just do not understand what can cause the pads to be knocked away from the rotor. I also had the same problem last year with 10" drums in the rear so I doubt the rear brakes is causing the problem.

I'm running 8x15" wheel with 4" backspace, is it possible that the problem is caused by the knuckle/ spindle deflection?

I know it will just be a band aid fix but will installing a 2 PSI residual pressure valve fix the problem?

I'm pretty frustrated by this problem and I'm about to rip off all the SSBC parts and change the knuckle/ spindle to the lager diameter disc brake spindle and buy the Dr. Diff 11.75" disc brake kit (stage 2)

Please let me know if any of you have any idea what could cause my problem.

Your rotor is flexing during hard use, a dial indicator will not see that.

SSBC is known for issues, but I haven't hear this issue before. As for the 2psi residual valve, all that will do is cause the brakes to drag 100% of the time, though it might mask your problem.

This is way out there but see if you have an interference between the caliper and any other suspension/body part at any point of suspension travel/position. I had a case once with a motor home that after a hard turn (can't remember which way), the first brake application wasn't there. What had happened in this case was the travel stop had broken off and when you turned, the caliper struck the frame and depressed the piston. Again, this is way out there.

Loose wheel bearings will aplify it too. My wilwoods have knockback,, they claim piston return springs are most effective.

Please torque the wheel bearings using a beam style torque wrench to 100 inch lbs. INCH lbs NOT FOOT lbs


After you do that please report your results
Signed
Hi speed Technical Support

Please read the factory service manual on how to PROPERLY adjust your front wheel bearings.

Right out of the 73 FSM for disc brake cars.

Adjustment
(1) Tighten wheel bearing adjusting nut to 90
inch-pounds while rotating wheel.
(2) Position nut lock (Fig. 10) on nut with one
pair of slots in line with cotter pin hole.
(3) Back off adjusting nut lock assembly one slot
and install cotter pin. The resulting adjustment
should be zero (no preload) to .003 inch end
(4) Clean the grease cap, coat inside with wheel bearing lubricant (do not fill) and install.
(5) Install wheel covers and lower vehicle to floor.

Thanks a lot for getting back to me with different alternatives that could cause my problem. There is absolute no interference between the caliper and the body, wheel or control arms. Wheel bearings are set ut just like the good old OEM service manual describes.

After discussing on e mail with Cass at Dr. Diff (super helpful with great knowledge) he could only se one part in my brake system that was not right and that was my prop. valve. He suggested to switch to a basic distribution block that move the same amount of brake fluid and pressure to all four calipers. I will change that and see if there is any change in the system.