spongy brakes

[Anthony L.]

I have a 1969 big block (L-68) with power brakes. Over the past few months I have had a recurring problem. The brakes begin to feel a bit soft or spongy, and eventually the brake light comes on when I use the brakes. Early on, the light would stay on. Recently after having done some work on them, the light hasn't stayed on, but the brakes don't feel as strong. The light will come on for a moment and then go off. I have had the master cylinder replaced twice (it currently has a correct (as original) master cylinder on it. I have had leaks repaired through replacing the calipers and rotors. I even had the proportioning valve replaced most recently. Basically everything has been replaced and still they go soft, though there is no recognizable leak anywhere. I understand that sometimes with dot 5 fluid air can develop in the system. I'm using the old dot 3. Does anyone have any ideas as to what I could even look for?!! Thanks, Tony

[Bill W.]

Re: spongy brakes

[quote) Basically everything has been replaced and still they go soft, though there is no recognizable leak anywhere.

Anthony, If you or a prior owner have drilled out the rear rivits on the rotors they may have been miss-indexed when installed. If so this can cause air to be pumped into the system by the rotor wobble that now exits. Get a dial indicator and check runout on all four wheels. If you find much over .005 you are pumping air into the system. If this is the case try re-indexing rotor on stub axle and recheck runout till you find the best (least) fit. Let us know what you find. Bill

[Jim T.]

Re: spongy brakes

Anthony from reading what you have done to your car in replacing the discs/rotors I assume that the rear units are the problem.
From your given history of what is happening and what has been done you are not going to like what I recommend.

Remove the rear spindles and ship the spindles and discs/rotors to Bairs.com. Bairs will rivet the rotors to the spindles (just like your 69 came from the factory) and true them as an assembly.

Installed back in your car car, no more air pumping due to excessive runout.

Best deal to make your 69 road worthy for many, many miles just remove the rear control arms and let Bairs do their professional rebuild treatment.

No worry for rear wheel bearings, new emergency brake parts, new front rubber bushing, and once installed and brakes bled properly, good brake pedal and excellent braking returns to your 69 like when it was new.

Most shops cannot duplicate Bair's service.
Problem solved.

[Anthony L.]

Re: spongy brakes

The mechanic that works on my car is a guy who specializes in Corvettes. That is all he works on in his shop, and has worked on them for 20 years. He assures me that it is the front brakes that are the problem. He says when there is a leak in the system or air gets in the system, it causes the proportioning valve to toggle over which allows the back brakes to be the primary brakes for the car. We have replaced the rotors in the front because they were either worn enough or there they were off enough that they needed to be replaced. I'll ask him again about the rear brakes, but he seems certain the problem is in the front.

[Paul J.]

Re: spongy brakes

Anthony:

It sounds like Bill and Jim have some experience in this. But if you want to persue the front brakes, ask your mechanic to change the hoses. They deteriorate from the inside and collapse. I don't hear much about this with vettes, but GM trucks are infamous for this.

Paul

[Jeffrey S.]

Re: spongy brakes

Anthony:
The earlier diagnosis of air pumping in the rear calipers is most likely what you have. Rebuilding the rear trailing arms is probably the best but most expensive way to fix the problem. Many years ago I was getting the same thing on my '69 and I replaced the pistons and seals in the rear calipers with o-rings seals instead of stock factory seals. It was very easy to do and not all that expensive. Florida Precision Calipers (I think that is the company name) sold me just the rears and that was the end of the problem. Some will say that I just hid the symptoms but didn't solve the root problem (rotor run out) and they would be right but my brakes have been solid since then and I'm happy with the results. To diagnose the problem I put the car safely on jack stands, started the engine and put it in gear. I applied the brakes and got very little response at the rear wheels. The system was obviously loaded with air. Did the fix and tried again and the pedal was firm and the wheels stopped. Haven't had to bleed the brakes in years. Hope this helps.
Jeff

[John H.]

Re: spongy brakes

The mechanic that works on my car is a guy who specializes in Corvettes. That is all he works on in his shop, and has worked on them for 20 years. He assures me that it is the front brakes that are the problem. He says when there is a leak in the system or air gets in the system, it causes the proportioning valve to toggle over which allows the back brakes to be the primary brakes for the car.

Anthony -

Your car doesn't have a proportioning valve; it has a distribution block which contains the differential pressure warning switch for the "Brake" warning light in the cluster, and it has no valving function at all.

The symptoms you describe are "classic" 1965-82 rear caliper air-pumping, resulting from excessive rear rotor lateral runout. I'd focus on solving that problem.

[Edward J.]

Re: spongy brakes

Anthony, Rotor Run Out Is The Biggest Problem With Older Corvettes, The Way I Check Is To Pull All Wheels Off And Crack Each Bleeder One At A Time, Air Will Surface To Indicate Which Caliper Has The Air,the Problem Maybe More Than One Rotor Is Causing The Air Into System.

[Tom L.]

Re: spongy brakes

MMM, brakes on old cars can seem like black magic. Well not really, I'll tell you the stories of two old cars with spongy breaks.

1. My long sitting, dare I say it, "Mustang" was the same way. I checked, replaced or rebuilt the entire system but to no avail. Someone suggested that I THOUROGHLY bleed and replace the fluid. problem solved. I guess that some fluids do fail (in this case, prbably moiture).

2. Fast forward to a few years ago when my wife convinced me to buy a corvette. We test drove it, no breaks!! After negotiations we bought the car anyway. The system was checked for obvious problems so I decided to use past experience. Changed the fluid and the brakes worked well.

It may be worth a try. Good luck!!

[Scott S.]

Re: spongy brakes

I had the same problem on my 72. I found the LR caliper (replaced by the PO) had .018 of runout, I indexed it one stud and it dropped to .003.

I also rebuilt both rear calipers with an o-ring conversion kit. (currently on sale at vbp)

The problem appears to be resolved....on to the next one!