I have good brakes... after 20 minutes of driving
I have subscribed to Auto Restorer for many years and have solved many problems by reading the “Mechanic On Duty” column. Here is my issue: A friend of mine who garages his 1968 Corvette in Brooklyn, New York, asked me to help him solve a perplexing power brake problem. Here it is in pretty much his own words: I use the car very little and it generally sits, particularly over the winter, for 30 days without being started— and when I do use the car, I will run it for 30 to 45 minutes. The annual mileage is no more than 250 to 300 miles.
When I start the car and begin to drive, my brake pedal goes to the floorboard and I have about 10% braking power. I’ve tried pumping the brakes, braking hard, braking soft, all in an attempt to get normal pedal height and brake
response, without success. I continue to drive the car with the pedal going to the floorboard and after 15 to 20 minutes of driving, normal brake operation returns. The pedal height returns to normal and the brakes resume a firm feel and 100% braking power returns.
I changed the master cylinder and brake fluid, and followed the correct procedure for bleeding the lines, but the problem persists. There are no visible leaks under the car or on the inside of the wheels, and the master cylinder remains full. There do not appear to be any vacuum leaks.
First, I must say that driving the car for 15 or 20 minutes with the pedal going to the floor and only 10% braking is a terrible idea for very obvious reasons.
The symptoms you describe usually indicate a failing master cylinder but since a replacement master cylinder didn’t improve braking performance that probably is not the source of your problem.
You state that you followed the correct procedure for bleeding the lines but I’m not convinced that the entire system is devoid of air. The rear calipers can be particularly difficult to totally purge and if the bleeder is in the wrong position you’ll never get all the air out. The rear caliper castings are normally drilled and tapped for bleeder screws at either end but only one bleeder position is correct. The bleeder should be toward the front of the car, so that it’s at the caliper’s highest point when the caliper is installed on the car.
Another issue with 1965-82 disc brake equipped Corvettes is that certain circumstances will cause the brake system to ingest air even when no fluid is leaking out and there are no apparent leaks that would allow air to enter.
When a car sits idle for extended periods of time the caliper seals can dry out and shrink, allowing air in. New seals will alleviate this problem to some extent but the only full solution is to use the car or at least use the brakes more frequently.
If a rotor is warped it will cause the caliper’s pistons to pulsate continuously as the car is driven and this pulsation will draw air in. Similarly, if a rear spindle is bent or if a wheel bearing is bad it can cause the rotor to wobble as it rotates and this will also cause the pulsating pistons to draw air in.
Use a dial indicator to determine if one or more of the rotors is wobbling excessively. Anything more than a few thousandths of an inch of run-out is too much. After the car sits for a while bleed
each caliper, taking notice of whether any have air in them. Have someone gently but firmly push the brake pedal and then open the bleeder on the right, rear caliper. Take note of whether you get a little burst of air out before the fluid comes out. Repeat this for each caliper and the master cylinder. If you get a puff of air out of a particular caliper or the master you’ve narrowed down the probable cause of the soft pedal. Put a fluid pressure gauge on the master cylinder and on each caliper line to determine whether the master is delivering sufficient pressure and whether the calipers are receiving sufficient pressure. As a rule of thumb, hard braking should deliver at least 1000 psi to each caliper. If you don’t have enough pressure coming out of the master then either there’s something wrong with the master or with the mechanical linkage between the master and pedal. If you do have enough pressure coming out of the master but not reaching one or more of the calipers then there’s an obstruction in a line, hose fitting, or the pressure switch beneath the master.