Help! 1976 corvette brake pedal goes to floor?

[dominicjason]

I have a 1976 corvette that has been sitting for a year ( that's how long I have owned the car ), The car looks like it has brand new break pads all around. When I started the car last week the brake pedal went to the floor, I found no fluid leaking around any of the calipers so I replaced the master cylinder. have been bleeding the system for a few days and have a good hard pedal when the engine is off but as soon as I start the car, the pedal goes effortlessley to the floor. Do you think the power booster is bad? Is there any thing I should check first before replacing the booster?

Thanks
Dominic

 

[duke94]

A bad booster won't make the pedal go to the floor. Sounds like you still have air in the system. Did you bench bleed the M/C before installation?

 

[dominicjason]

Yes I pre, bench bled the master cylinder before installation, and I have been using the two person method for bleeeding the system. I see no sign of any bubbles exiting into my jar and clear plastic tube. Ill try this way one more time and if the brakes dont improve do you suggest a diffrent type of bleeding method?

Thanks

 

[GerryLP]

Yes I pre, bench bled the master cylinder before installation, and I have been using the two person method for bleeeding the system. I see no sign of any bubbles exiting into my jar and clear plastic tube. Ill try this way one more time and if the brakes dont improve do you suggest a diffrent type of bleeding method?

Thanks

I have used the two person method, and it works alright, but a definite improvement on my Vette's brakes was the use of the "Motive Products" pressure bleeder. However, it does come with 2 pieces of chain and 4 "J" bolts and wing nuts, but these are not as effective as using a piece of metal stock over the adapter plate and a "C" clamp. With this set-up, one can reach the 20 - 25 psi pressure stipulated in the shop manual. In fact, the chain set-up is not worth trying, for the mess you most likely to make. In fact, I have communicated with Marco, from Motive Products, and he was very appreciative of my feedback. He told me about the "C" clamp option. Because the 0105 model is a Universal model pressure bleeder, the chains don't work as well as it perhaps would for other chevy car lines. But the main pieces of the system do work great.

And the brakes work beautifully now...

The picture of the gauge is blurry, but that represents 20 psi. I also had it at 25 psi, but did not take a photo.

 

[dominicjason]

Thanks so much for the pictures, I just ordered a motive unit. I two man pressure bled the system for the third time, but this time I had a small floursent shop light held up to the plastic hose. There is deffinetley small air bubbles traped in the system. When my motive bleeded arrives I will take the master cylinder out of the car , rebench bleed it and try again. Im beginning to think the two man way is ineffective due to the master cylinder being divided. The minuite you start bleeding the front or back with some one pressing the brake pedal your drawing air that is traped in the lines into the master cylinder from the end of the car your not loosining the bleeder valuve on.

Thanks again

 

[GerryLP]

Thanks so much for the pictures, I just ordered a motive unit...When my motive bleeded arrives I will take the master cylinder out of the car , rebench bleed it and try again....
Thanks again

Dominic,

Congrats! You won't regret it. Two large bottles of brake fluid will fill it about half-way. Make sure you have enough in the bottle to avoid injecting air into the system. Also, it is great for flushing the brake system with new brake fluid.

You may not have to remove the master cylinder and bench bleed it. Really, the way to bench bleed a master cylinder is by using a fine clear tubing, but the plastic fittings leak (which is a source or air leak as well). I made a solid bench-bleeding tubing once, but the size was too big (due to the fitting size needed to cinch it down in each port), so this was a losing situation.

The pressure bleeder should get rid of the air bubbles from the master cylinder as well, but to make sure, after you bleed the brakes, release the pressure from the bottle carefully. AND DO NOT DISTURB THE SET-UP. Then pump the brakes through the brake pedal (with enough pressure to simulate a moderate stop) 2 - 3 times, and finally, set the Motive Products' bottle gauge to 20 - 25 again, and go around one more time on the brakes (left rear inner, left rear outer; right rear inner, right rear outer; right front, and left front). That should do it.

Remember that if while driving, you press the brake pedal, and if it tends to kill the engine, then you could have a bad brake booster. With a good booster, a gentle press of the brake pedal should make the pads grab the rotors and slow down the Vette from around 30 MPH at 1/2 way of the brake pedal (approximately; with new brake pads on).

Good luck!

 

[73shark]

Also it helps to rap the caliper w/ a rubber mallet which w/ loosen small air bubbles that cling to the insides.

 

[duke94]

One more thing to consider, if your caliper seals are old/worn then you will suck air past the lip of the seal during brake release and never get it bled out. How old are your calipers?

 

[dominicjason]

Thanks for the information, I dont know how old the seals and calipers are . I bought the car a year ago and I dont see any indication of brake fluid leaking from any of the calipers. Is it possible to not have any fluid on the caliper seals but air can enter. Is there a way to test the caliper seals or should I just rebuild them if my break bleeding attempt with the motive tool fails.

Thanks

 

[duke94]

Yes, the caliper seals will suck air but not leak brake fluid. If the brakes look like they have not been serviced in a long time, I would rebuild them or better yet get the O'ring style replacements and your bleed problems will be over. No harm in trying the Motive bleeder first. It always applys a positive pressure during bleeding where the pedal bleed procedure pulls a vacuum when the pedal is released causing the air to be pulled in past the caliper seals.

Also, buy a big C-clamp to hold down the Motive cap as the chains that come with it don't work well.

 

[dominicjason]

I just got the motive tool this weekend, it worked great with the C clamp method there were no issues. I bled the system three times, tapping on the calibers as I went. No air bubbles were seen exiting the system. The pedal still goes to the floor when the engine is on but is firm when I pump on it with the engine off. Do you think I should now focus on rebuilding or replacing the calipers. Could one of the caliper pistions be stuck?


Thanks

 

[GerryLP]

Dominic,

Now try the following.

With the engine running, try to listen for a hissing sound around the brake pedal area. You can use a piece of hose about 1/2 inch diameter, and use it like a stethoscope. If you detect a hissing sound, then first, make sure it is not coming from the headlight switch or the vacuum switch below the steering column which opens your head light doors without turning on the headlights themselves. If those two are not leaking, then it is definitely your brake booster (that huge round thing behind the brake cylinder :eyerole).

When my brake booster went bad, it was hissing, and whenever I pressed heavily on the brake pedal, it would make the engine run rough or kill the engine (probably because it was venting vacuum signal).

Now, the brake booster may seem hard to do, but I was surprised how relatively easy it was to replace. Just leave the master brake cylinder hooked-up (and of course hooked-up to the lines) and covered by the reservoir lid tightly. The two are only connected mechanically, and it won't affect your brake lines (i.e. air won't get in). The shop manual shows how to do it.

Good luck!

 

[raybies]

Bro
The most common problem with ALL vettes is the rigid mounted calipers.
If you have or if there has been rotor run out more than .004 to .008" what probably happened is it took out a caliper.
You won't see fluid leaking but it will suck air into the system.
You can chase your tail but what i would do is put a dial indicator on the rotors and if any of them run out more than the spec. have them cut and gently lift back the dust boot on the caliper piston to see if its seeping.

 

[dominicjason]

Thanks for the information, ill diffentley check out the rotors and seals. I just got a new brake booster in the mail today. Howevrer it will be a couple of weekends before I can get back to the corvette do to a project at work, but im looking forward to getting back to the brake problem investigation.

Thanks

 

[LanceB]

Dont mean to hijack the thread, but I invested in the Motive bleeder today. Hope to try it out when the weather breaks. I have replaced rotors, calipers, and my brakes were better (no more brake light coming on and the pedal going to the floor), but after sitting in the winter, they are always spongy and difficult to bleed the way I am doing it. I have also read that the wheel bearings can be worn which causes the rotors to be wobbly; so they can suck in the air. Any opinions on testing for that? Glad to read this thread!

 

[GerryLP]

Dont mean to hijack the thread, but I invested in the Motive bleeder today. Hope to try it out when the weather breaks. I have replaced rotors, calipers, and my brakes were better (no more brake light coming on and the pedal going to the floor), but after sitting in the winter, they are always spongy and difficult to bleed the way I am doing it. I have also read that the wheel bearings can be worn which causes the rotors to be wobbly; so they can suck in the air. Any opinions on testing for that? Glad to read this thread!

The owner of Corvette Carter's in Livermore, CA told me the same. I bought my SS Calipers from him over 20 years ago, and they still do not leak. You'll have to do a hub run-out, but I have never done it, yet I believe it is in the shop manual.

GerryLP:cool

 

[LanceB]

Used the new Motive bleeder tonight. So far very happy. the last bleeder let out a lot of air then nothing but solid clear brake fluid. Got bubbles out by tapping with rubber mallet. So far very tight. Wish I had a nice lift though. Having to lift it and put it all on jackstands can be a big PITA.