I get alot of cars in my shop with complaints of low brake pedal, and found that after all of the other usual causes are looked into, (brake adjustment, pedal ratio, air in the system,etc)one of the biggest causes is the use of GM low drag calipers on a car without a stepbore master cylinder.
The problem is that the piston in a low drag caliper retracts .020-.025" after the brakes are released. A regular caliper retracts about .005-.010"
You would not think that's much, but with a 1" bore master cylinder that .025" on each caliper turns into almost 1/4" of piston travel at the master cylinder just to get the pads to touch the rotor.
With a 6:1 pedal that is 1.5" of pedal travel before you even start to build pressure... Add the amount of pedal travel it takes to actuate the rear brakes and take upslop in the linkages, brackets etc and the pedal starts getting real close to the floor, especially with an underfloor pedal.
Using regular calipers will only use up less than half of that travel, leaving more travel at the pedal to build pressure.
I've checked a few calipers using a dial indicator, an easy bench test is to put a 2" block between the piston and caliper, then put about 30 psi of air in through the banjo bolt fitting...the block will keep the piston from coming out, and you can either put a dial indicator on the block, and see how much it retracts after you let the air off, or check the space between the block and caliper....If your brakes are installed already you can see if you can fit a.020" feeler gauge between the pad and rotor after you release the brakes. Typically,being able to see any space between the pad and rotor after release of the brakes can indicate that it is a low drag caliper. ( or you could have sticky anchor pins)
I have installed a 10# residual pressure valve in the front brake lines of a car with low drag calipers, this cuts the piston retraction back to about .015", but being as how most of the RPVs test out at less than their rated pressure, and most leak down, it is a shakey fix at best.
A good metric caliper to use with one of the disc swap kits is a 78-81 Malibu. I have found that each caliper from rebuilders needs to be checked, as many times the later calipers get put in the wrong box at the rebuilders.
Later, mikey
The problem is that the piston in a low drag caliper retracts .020-.025" after the brakes are released. A regular caliper retracts about .005-.010"
You would not think that's much, but with a 1" bore master cylinder that .025" on each caliper turns into almost 1/4" of piston travel at the master cylinder just to get the pads to touch the rotor.
With a 6:1 pedal that is 1.5" of pedal travel before you even start to build pressure... Add the amount of pedal travel it takes to actuate the rear brakes and take upslop in the linkages, brackets etc and the pedal starts getting real close to the floor, especially with an underfloor pedal.
Using regular calipers will only use up less than half of that travel, leaving more travel at the pedal to build pressure.
I've checked a few calipers using a dial indicator, an easy bench test is to put a 2" block between the piston and caliper, then put about 30 psi of air in through the banjo bolt fitting...the block will keep the piston from coming out, and you can either put a dial indicator on the block, and see how much it retracts after you let the air off, or check the space between the block and caliper....If your brakes are installed already you can see if you can fit a.020" feeler gauge between the pad and rotor after you release the brakes. Typically,being able to see any space between the pad and rotor after release of the brakes can indicate that it is a low drag caliper. ( or you could have sticky anchor pins)
I have installed a 10# residual pressure valve in the front brake lines of a car with low drag calipers, this cuts the piston retraction back to about .015", but being as how most of the RPVs test out at less than their rated pressure, and most leak down, it is a shakey fix at best.
A good metric caliper to use with one of the disc swap kits is a 78-81 Malibu. I have found that each caliper from rebuilders needs to be checked, as many times the later calipers get put in the wrong box at the rebuilders.
Later, mikey