Big brake kit problems 1970 Chevelle

CPP, I have your Big Brake kit on my 1970 chevelle.
Your part number: 6872FRBK-BB
From the start I have problems with it, can you help me?
I have a 7” Dual diaphragm booster on the car, enough vacuum.
The car does have 17” wheels with 255/50 front and 275/50 rear Nitto tires.
I suspected the booster pin, and adjusted it (also for testing) to tight, to loose and just spot on (it was good, just changed is to test). It does not have any prelaod on the master cylinder.

I checked everything to find the problem. Maybe a wrong master cylinder, or one with a valve in it? No, it looks like the right one. No valve and a 1” bore diameter. Bench bleed it a couple of times just to be sure that’s not the problem.
Maybe a wrong proportioning valve (for drum brakes)? Did check it, also the right one.
Caliper Alignment? Did check it over, and over again. It’s perfect.
Preload on the brake booster? No, pedal adjustment is good, properly align, pedal ratio is about 4:1

I’m 100% sure that there is no air in the system, no where. I did try 3 bleeding methods.
gravity bleeding / vacuum bleeding / pedal pumping,bleeding with helper. There is no air, can drain or pump fluid trough the system many times.

By the way, your instruction for the rear calipers are NOT the ones for this calipers. The e-brake system on this calipers is different.

Problem 1:
Passenger rear caliper drag. From the start there was a lot of brake dust on this wheel.
Checked it after a 10 mile drive, 40 and 30mph stops. Front discs are 90 degree Fahrenheit, drivers side rear 85 degree Fahrenheit, passengers side rear 160 degree Fahrenheit and a lot of dust. Brake was dragging very bad.

And the passangers side caliper was machined to deep! I needed 2 copper washers when I mounted the brake line. The caliper is machined deeper then 1 washer is thick. (no brake line was not upside down). ;)

I thought about everything, e-brake, preload, but could not find a problem. Why only one caliper? After many nights of searching I opened the caliper to see what’s wrong. Could nog find anything, assembled it and made a test drive. Without any result. I did have a feeling is was the e-brake system that was locking the piston for clearing the brake pads after braking (e-brake or normal foot brake). Even though on some test drives the cabels were disconnected. Finally I just needed to know what it was, I even disassambled the piston and drilled the thread out of the e-brake system.
Put everything back together, bleed the brakes, pumped the pedal a couple of times to get the pads to the disc and made a test drive. It drove great!!! No more dragging!!! Problem found, now I ask you for a solution?
Do you have a new working caliper for me? Or parts to rebuild a new one that’s broken from day one?


Problem 2:
Locking rear brakes on hard stop.
When I make a emergency stop, or brake really hard the rear brakes lock. Both at the same time. The front doesn’t. Did try it a couple of times with the same result. Even with the brake pedal almost on the floor, the front does not lock and the rear is making 2 black marks on the road. I can put a adjustable proportioning valve in the rear line. But when that is necessary, why is it not in a ‘complete’ kit?



Problem 3:
A bit soft or spongy brake pedal, don’t say there is air in the system.
I know how it looks, no locking front brakes and a spongy pedal is air in front brakes.
No, there is absolute no air in the system, even not with calipers on there side to bleed the system or with the pistons pushed back. Brake pedal takes force, car brakes good, but for my feeling the pedal is a bit deep, soft or spongy.
I do have a normal rubber brake lines, not the braided ones but that can’t make so much differance does it? Did read a lot about the front calipers giving this problem with truck owners (were the calipers are on original).

I have a C10 pick up, a G10 van, both 1977 and a K10 suburban from 1979.
All 3 original, discs front, drums rear. Original booster and 1 1/4 master cylinders and they have a much higher but also harder brake pedal.


Can you help me, problem 1 is my biggest problem. I finally have a driving car again, but do not get trough the inspection we have every 2 year (it need e-brake on both rear wheels).
Problem 2 is strange for a complete brake kit and problem 3… well maybe it’s just the way you designed this system.