Trying to solve soft brake pedal problem on 1976 vette.

Car sat for about 2 years now has a soft brake pedal. I drove it a few weeks then bled all callipers. The front callipers were leaking so I replaced both of them. Bled all 4 callipers but still has soft pedal. It nearly goes all the way to floor.

Last weekend I rebuilt the master cylinder. Bled the master cylinder, then bled all 4 callipers using a hand vacuum pump. Without driving the car the pedal is still soft.

Anyone else have similar problems? I saw some threads talking about rotor runout. Would that create this problem if I haven't driven the car yet but the pedal is still soft?

Thanks

Roger

 

I feel your pain...the brake systems are evil on these cars. I bought my '76 from someone who could no longer drive and the car sat for a couple of years before he decided to sell it to me. As a result, the brakes were leaking and needed lots of attention. So I rebuilt the calipers, replaced the pads, replaced springs and shoes for the parking brake and replaced the rear rotors. Front rotors were fine so I left them as they were.

I have a vacuum pump bleeder tool that I've used for years on all sorts of cars, never had a problem. For whatever reason, it just didn't work well on the Corvette brake system. Seemed to work, but when I was done, the brake pedal was still soft. In the end I had to have my son come out to the garage and help me bleed the brakes the classic way. When we were done, the pedal was firm and brakes worked fine.

It's been over a year now and the everything still works great.

Just throwing this info out there in case it helps you out somehow. Good luck partner!

 

Check the vaccum lines coming off the carb.

 

with Bronz's assessment! I think the volume of fluid in the Corvette system is too much for the small volume of air/fluid the Mityvac can pull per stroke. Manual bleeding works fine for me. I have found from experience that Speed Bleeders really help the process. Twice around the car and I was done. The brakes are still firm and stop the car well after a couple of years of use.

 

I just bled all 4 callipers manually. I had clear tubing attached to the bleeder as I opened them. Didn't see any air come out of any of them. My pedal is still very soft and goes nearly all the way to the floor. I bled about 4 to 6 ounces of fluid out of each calliper. Does that seem like enough to make sure there's not air in the system?

Thanks for the advice, please keep it coming, I'm not giving up!

 

go through the archives there is a lot of this there. if it sat for 2 years then you need to go through the system. If you replaced the rotors the runout must be under 005 or you'll never get a firm pedal. The hoses should also be replaced and the bearing endplay checked.

 

Just asking the obvious question, which I ask everyone that calls: Did you bleed from all 6 bleeders?

 

Most of the time it's air in the lines.I gravity bleed mine first and then use a vacumm bleeder.sometimes a few taps on the caliper with a rubber hammer helps get the air to the bleeders.Good luck!

 

Mine sat like that and I did everything possible and still had soft pedal. The guy (mechanic) helping me kept disagreeing with me that it was the brake booster. He said he had never seen one go bad on a Vette. I asked him to humor me and three years later I am still driving with plenty of pedal and a new booster. Don't discount it.

 

There's gotta still be air in there somewhere. Manual bleeding has worked for me after replacing calipers.
Others have mentioned the brake booster, but i don't see how it can cause a soft pedal that goes nearly to the floor. An inoperable booster will cause a very hard pedal and really poor braking.

 

I had the same problem on my 75 vette that sat for 17 years before I purchased it.

I went through almost two quarts of brake fluid before I noticed a small leak in two of the

four calipers I had rebuilt. I hadn't changed the pistons just the gaskets. I was told that the pistons must really be smooth and true before you start. Mine weren't. The leak was very slight. I got frustrated went to Kragen and bought two rebuilt calipers. That cured the problem. After purchasing the calipers, I gravity fed all four calipers, had my son start the car and pump each bleeder which had a clear hose attached and fed into a clear glass mug 1/4 filled with clean brake fluid. It took me forever, but in the end, I have really firm brakes now.

Good Luck

 

SH, you are right. It's been awhile since I did the brake work. I started out with soft pedal after replacing my calipers and bled them. By the time I was done I ended up with a hard pedal and it was a bad booster.

Good call.

I'm getting old and can't recall things like before!

 

MUSKEGON BRAKE

NO. Where are the 5th and 6th bleeder? This could be big!

Thanks

 

Last night I removed the 2 brake lines from the master cylinder and installed plugs in the master cylinder. The pedal was very hard so I don't think I have a brake booster problem.

Hooked the brake lines back up to master cylinder and started bleeding the callipes. I bled the 2 front callipers by unbolting them from the spindles. I did every method 1.)started with gravity bleeding, then had my son pump the brakes as I bled them, then used my hand vacuum pump. If there's any air in left I left the brakes hoses connected but turned the callipers so the bleeders are at the highest point of the calliper assembly. I also stuck wood blocks between the calliper pistons. I did get some air out them but my pedal is still soft.

Tomorrow I will bleed the back callipers again.

 

Beg, borrow or steal a pressure bleeder.

There are a few good reasons why pro brake shops use them.
Seen them as low as 45 bucks. Well worth the aggrivation and time you have spent dicking around with this.

To flush and bleed (recommended), I usually use about 2 quarts and don't have the bleeding problems commonly discussed here.
Older fluid is usually darker, but when replacing new with new, just measure the in and out quantity to flush. (This will also tell you there cannot be any bubbles left in the system).

BTW, yours is the best test for a master cylinder
FYI, as mentioned earlier a bad booster will always give a hard pedal, not soft.
Transferring your caliper back from the wooden blocks to the rotors will give enough piston movement to loose pedal all by itself. It doesn't take much. I use the blocks too, but not to bleed.

Example:
Take a piece of small clear hose. Fill it and leave some air bubbles in it.
Drape it over something higher gradually so you have a w shape with the ends up. Lower one end a bit to drain a little and watch the air bubble at the highest point. The water will still flow slowly and leave the bubble there. (Same as a water level)
Do the same thing with pressure and the bubble will be blown thru.

 

The rear calipers have two bleeders each, one on the inside and the other on the outside of the caliper. There is also a bleeding sequence to follow, but I don't have that info handy. I have replaced everything on my brake system except the booster -- even shimmed the runout on the front calipers following GTRR1999's posts. Brakes feel perfect now!!!

 

If you have done everything except the other 2 bleeders on the rear,i would think that would take care of your soft pedal.

 

Bleeding order should be furthest from m/c to closest, and you can do just the rear circuit. Should be RR inboard, RR outboard, LR inboard, RR outboard.

 

I have to wonder if you recently bought your 76 after it sitting for 2 years? I'm assuming the car has stainless steel sleeved calipers. What we don't know is, what kind of brake fluid you're using? Dot 3 fluid or Dot 5 silicone.

Back in 1980 I put stainless steel calipers on my 74, rebuilt its original master, flushed the original steel lines, new hoses and used silicone brake fluid. I had a soft pedal that would go to the floor if pushed hard enough. But stopped the car (at any speed) at half to 3/4 the pedal travel to the floor.

1979 bought 4 new calipers from GM for my 65 (couldn't afford SS at the time). Did the job and in 1986 these calipers leaked. The car sat until 1995 and finally bought the SS calipers. Did the job to my 65 except upgraded to a dual master cylinder, new hoses, new steel lines and silicone brake fluid. Same thing as my 74, the soft pedal syndrom.

2 months ago. On my sons 77, new SS calipers, new SS hard lines, new hoses and a new rebuilt master (original rusted inside and out). His pedal does what the 74 and 65 do. It took me a long time to convince him, this is what it is, when using Dot 5 silicone brake fluid.

Silicone fluid makes for a softer pedal. It'll still lock all 4 up.

Start your 76, put it in drive (up on jack stands of course) and simulate a stop. The rears will stop. Get a socket and breaker bar on one of the front lug nuts. Turn the front wheel with the brakes off. Have your son hold the brake pedal and then try and turn the wheel/tire with the breaker bar on the lug nut. If you can't turn either of the front wheel /tires, you're good. But don't go crazy on the lug stud, you don't want to break it off.

 

It is true that silicone brake fluid is more compressible than the DOT4 or DOT5, but a lot of people never notice the difference. Having that soft of a pedal because of fluid choice would be unusual, but not impossible.