C2 - 66' coupe brake troubles

[Dan Pepper]

Problem: Over a period of driving about 300 miles, both front and rear brakes loose effectiveness at an equal rate until the pedel bottoms out, and no brakes. On bleeding the brakes again - the second time in about 600 miles - more air was pumped out of the lines.

Last year, I had the car's J50 MC rebuilt (by Whitepost) and also replaced front/rear calipers (w/speed bleeders), new pads and hoses.

The front brakes prior the the maint. were new/stainless sleeved units and worked fine, the rear calipers were the original '66 units and were frozen/shot. I "upgraded" the rear calipers with stainless sleeved service units. The rear hub assy has never been separated, so run-out is not a problem and there is no fluid leakage evident anywhere.

What would cause both front and rear brakes to go soft at the same rate? The only commonality between front and rear lines is the MC, but since it was rebuilt/resleaved by a reputable rebuilder - I'm thinking the problem must be elsewhere - any ideas?

[Joe L.]

Re: C2 - 66' coupe brake troubles

Dan------

Given the symptoms which you describe, air is entering your brake hydraulic system. The only 2 places that it could enter is at the calipers or the master cylinder. Since you have power brakes and a dual master cylinder and since you are getting air into both front and rear circuits, I doubt that the master cylinder is your source of air in the hydraulic system. The power brake booster could NOT be a source of air in the hydraulic brake system.

You cannot assume that since the rotors have never been seperated from the hubs/spindles that excessive runnout does not exist. You need to dial indicate the runnout on all of the rotors as a first step in determining the problem. If it's greater than .005" on either of the fronts or either of the rears, that's your problem.