ok ... thanks ... a point of clarification, though.

You are using bore diameters in that calc. But isn't that assuming a similar volume in each?

In the MC there is a ~2" long tube of fluid that the MC piston rides in. In the caliper, there is only 1/8" of the piston bore filled with fluid with new pads. The rest is displaced with piston itself.

So it looks to me like the ratio would be reversed from 3:1 to 1:3 due to more volume being displaced for a given length of travel.

Help?

And I don't know where else to look for knockback. I've even replaced the C-clips in the rear end ...

I was just thinking about all I've done ...

The one thing I haven't done is lubricate the rear guidepins. If they were binding, could that be the cause?

 

It's the change in volume that's important not the total amount in either the MC or the caliper. IOW the piston movement times the piston area. The end of the piston is a circle and the area that I'm refering to is pi*(diameter/2)^2. So the volume moved is that area times the stroke of the piston.

Assuming there's no brake line swell or air in the lines and ignoring any ABS activation, the volume of fluid that is pumped out of the MC when you move the brake pedal is equal to the volume of fluid pumped into the calipers. There's no place else for it to go. The volume of fluid that is pumped from the MC to the caliper and back when you release the brakes is the amount that is pushed out of the MC bore by the MC piston's motion. Since the MC is a smaller diameter and hence a smaller area than the Caliper pistons it has to move farther to make up for its smaller area.

Think of it like engine displacement. To get the same displacement from an engine with a small bore as an engine with a larger bore, you have to have a longer stroke.

The MC is like the small bore, long stroke engine and the caliper pistons are like the big bore, small stroke engine. Not to mention there's four of them that have to be feed by one MC.

The MC is designed to be smaller diameter than the caliper pistons in every brake system that I'm familiar with. This is done to produce mechanical advatage...IOW the force squeezing the brake pads together will be higher than the force you are pushing on the MC piston with. This force multiplication plus the added help from the vacuum booster makes stopping the car possible for those of us with right legs skinnier than Arnold Schwartzenegger's.

As far as knockback is concerned, even with new c-cips there will be some play of the rear axles. Same thing for front hubs. Each of these could acount for knockback. Lubing the guide pins in the rear may help some, but since the Prospeed calipers are fixed any play or deflection in the front hub will produce knockback. You may have to live with some of it.

It curious that you only get it in only one place on the track. That might be the key to diagnosing what component is responsible for the knockback.

I have the opposite problem. I get a very hard pedal after flat out left handers. The pedal doesn't budge hardly at all and the the car does not slow down at all. Very exciting! It feels to me like the booster is losing vacuum in these flat out left hand turns. If you have every had a booster go bad on you, you know the feeling. It's a religious experience.

 

Thanks ... I'm gonna ponder on it a while ...

 

The pads were new. I'll check to see how they are wearing....

 

Can't quite understand that one. The pedal shouldn't feel soft or spongy like air in your brake system. It should just feel like you have longer travel before the car starts stopping. Once the pads fully contact the rotor though the pedal should be firm.

 

"Knock back" seems to be the exact description of what is happening here. I have had the same issue , different brake pedal heights and effectiveness after different corners which improved 100% with replacing the front hubs. The worn movement may be only a few thou but at the outer edge of the rotor it is much larger and easily enough to push the pistons back into the caliper.

It was certainly very disconcerting ( L side worn more than R) where a L turn would leave the pedal at one height and while approaching another corner after a R turn the pedal would require 2 pumps.

BTW this problem couldn't be air as that would produce a consistantly low pedal .

Also I think the rears take so little volume compared to the fronts that C clip wear is not a major issue.