I had a pulsating brake pedal in my daily car.

LRO
Lateral Runout of a Disc Brake Rotor should be at or near .001" on a C3

You need this flexible arm dial indicator tool to measure lateral runout on brake rotors

My runout (LRO) was in spec, but I still had pulsations. How could that be?
So I dug deeper.
And all the time I was thinking how a lot of this applies to my C3.

DTV:
Disc Thickness Variation. I measured the disc thickness at many locations, and found it varied by 0.0018, 1.8 thousandths. The GM spec is 1.0 thou max, 0.4 preferred. That is the source of my pulsating pedal. My new rotors only varies by 0.4 thou. You do need a good 1-2" micrometer, than reads to a thousandths, and then has a vernier scale to read to 0.1 thou. I'll be checking the C3 too now. Most people call this a "warped" disc.

Stop the dripping brake line:
This is a new one I just found. When you take the calipers off, it is always a leaky mess right? No more!. Push the brake pedal an inch or two, and it closes the vent hole in the Master Cylinder, and if air can not get in the top of the hose, gravity will not let it leak out the bottom of the open flex hose. Not by gravity anyway! Sounds simple, but it really works. I have always been concerned about causing internal brake line damage by pinching it shut, to prevent causing a mess, but now you do not have to do that.

That is all it takes to stop the brake fluid from dripping out of the open flex hose, once the caliper is off.

An open Flex hose will not drip while prop is holding the brake pedal.

Testing for flex lines blockage
The above trick works in reverse if you loosen the prop on the brake pedal. It should start to flow immediately and steadily. Several drips per second means you do not have a blocked / collapsed flex hose.
Brake line with steady drips confirms no hose blockage.

Testing for stuck caliper pistons aka Caliper Pull Back Test
Caliper pistons rust, and become sticky, causing a variety of brake problems. Like "warped" rotors. Our C3 calipers are notorious for rusty calipers. This test will let you check for corrosion without caliper dissasembly. Watch piston closely. Apply brake pressure, then release, then repeat.
Calipers with square o-rings, very common on new cars, pull the piston back .004-.008" when brake pressure is released. You can just see the movement if you look very closely. I tested these on the bench with 50psi of air pressure, then released it. I also dissasembled one of these and it was clean as a whistle inside. Fluid has been changed every 4-5 years since new. Yes it has iron calipers. So that confirms changing the fluid regularly keeps the iron piston corrosion at bay. This car has a 2 thou LRO spec, and with that much piston retraction, I can see how 2 thou will not be a problem, but it is in our C3s.

C3
Lip seals: The C3 calipers have different seals, not a square o-ring. The lip seals will not pull the pistons back. With the pads touching the rotor constantly, I can see why over 1 thou runout causes problems.
O-ring seals: The popular C3 o-ring conversion uses round o-rings. Someone needs to test this and see if those seals pull the pistons back.
Wilwoods: Use a square piston o-ring seal. So they would definately do this and can be checked. This feature should also make them much more tolerant to a little rotor run-out. That is another advantage to them that I have never heard anyone mention.

 

Thanks for taking the time to write this up. A lot of good information that is described in an easy to understand way. And you included pictures!

 

As per the O ring conversion calipers. Due to the piston design, the way the O ring is retained. It does not twist up and pull back. And with the constant pressure of the springs behind the pistons, it wouldn't matter if it tryed.
So, no piston pull back. Nice write up by way.

 

Thanks 4-vettes.
The square o-ring design is "stiff" enough that it took me ~30lbs of force to slide the piston in either direction.
Stiffer than I expected. New ones or old ones were the same.
I read the 6 thou of pull-back is just seal flex, not actually sliding.
The square seal has say ~8 thou of clearance, and just bends to the left a little, then springs back, during normal brake application.
It does not actually slide until it has to compensate for wear.

I'll have to test a C3 lip seal caliper soon. Curious minds want to know!

 

Thanks @leigh1322 !