I usually don't drive my Corvette on long trips, but I was moving and decided to make the 500 mile drive in my 1979 L82 with 27,000 original miles. I was at Highway speed and was exiting the Interstate when as I braked, the brake light came on and the brake pedal went soft and nearly to the floor. I stopped and added brake fluid, though it wasn't really low. Still soft. I went to an auto parts store and replaced the Master Cylinder Reservoir gasket. Better, but still a little soft and the light remained on. I then made the bails that hold the cap a little tighter. The brakes seem much better, but the light stays on. When I pump the brakes hard, it goes off, but comes back on with additional braking. No leaks detected around the master cylinder or the wheels and it does not seem to be loosing brake fluid either. Brakes work well, but I still can't get the light to stay off. Any suggestions please.

 

your rotors have runout, which upon sustained high speed driving , causes your calipers to cavitate, eventually pumping air into your calipers.. your car being that old with that low of miles is a likely symptom.. the brakes are just not exercised enough to keep the seals oiled, they dry out and let air in, and probably some fliud out.

you will need special calipers that resolve this issue. they are built with O-Ring seals and are less prone to letting in air.. and you will need to exercise your car more often.

the gasket you mentioned has nothing to do with letting in air. it only keeps fliud from splashing out.

and as a result , your brakes cannot be working well.

 

Bleed the brakes.

 

yes bleed the brakes, that should do you until the next road trip..

 

Thank you both for your suggestions. As much as I would like to overhaul the rotors and calipers now, I will first bleed the brakes to see if that will solve the immediate issue until next spring when I can spend more time and cash on the brake system. I appreciate your replies.

 

It's unlikely to be rotors unless somebody has already fooled around with them. I'd look for worn rear wheel bearings.

 

I had a similar problem that nobody could diagnose. In my case it was not that the rotors had runnout or warpage, because I'd even replaced these to hopefully cure the problem....it didn't help. I still had to have the corner garage bleed the brakes every few weeks....what a pain. Had them put on new master cylinders....no help. They pulled a vacuum on the whole system to suck out every bubble of trapped air....no help. Checked for brake fluid leaks around the whole system....no leaks. The experienced corner garage mechanic finally gave up.

Took it to a Corvette shop here and the owner new right away what the problem was.

So the Corvettte shop owner said, 'your wheel bearings are shot, even though you don't necessarily hear it or feel it. This causes the rotors to vibrate very slightly during hard stops, like at a tollway booth. You won't feel it, but it's enough to suck in a minute amount of air into the lines through perhaps slightly worn caliper seals.'

I had their shop replace the wheel bearings, one spindle, and set up the rotors and rebuild the calipers, redo the parking brake while he was in there, too....They used a special fixture to set the bearings and spindles and turn the rotors keeping it all together as a unit, then installed them all back as a matched unit. problem solved. I think it cost about a grand all told, but I was ready to burn the car and collect the insurance before finding the cause....so I bit the bullet and had it done.

Even got a car wash 'free' thrown in by the shop for only a grand!

I've owned three mid seventies 'Vettes in the last ten years and drive the hell out of them. I don't care to work on them any more with my limited space and tools and time. And I've found that there is a night and day difference in diagnostic ability and skill between a corner garage, or even a Chevy dealer, and a true Corvette specialty shop.

I'd say ninety five per cent of the problems I read about and which drive us nuts here on the forum are caused by bad guesses and inexperience by amateurs or even professional mechanics who rarely work on C3's in their daily business.

The 'Vette shops charge a hundred bux an hour typically, and while no one likes to pay for that, myself included, you get what you pay for.

 

Terry,

Perhaps your idea explains something else I have noticed. If the Wheel Bearings are bad, is it likely that I might hear a light thumping from the rear during slowing and cornering together? I know the rotors and calipers are the originals, but they still look very good. I will eventually rebuild these, but based on priority. If the wheel bearings are needing replacement and that can effect the brakes as you describe, I will plan to replace them soon. I will first go ahead and do the bleed this week and see if that helps.

Thanks for your input.

 

I had the EXACT thing on my 78. It is not runout....not bearings....it is worn seals in one or more calipers letting air in. If your calipers have never been replaced I would suggest replacing all as I did. One of my rear calipers was really worn and in bad shape. You can bleed all you want and the light will come back on the first time you have to hit your brakes hard. I bought a package from Ecklers. You may choose to buy the o-ring type but I chose the original lip-seal. It is not a hard job...but take your time. By the way. I gravity bleed my brakes....works great every time.