Soft brakes on 1981 Stingray

[tshera388]

Hello all, Long time reader, first time poster. I know this subject has been addressed several times in this forum, but every one seems a bit different so I'll add this one so everybody can keep score.

The problem is the usual, brakes soft and pedal goes to the floor. I've bled the brakes to the tune of 2.5 quarts of fluid with no real results as far as pedal goes. I did flush out some sort of contaminate that separated from the fluid when standing in my recovery jar. I have replaced the master cylinder. The booster seems to be functional and holds vacuum. When the brake pedal is pressed hard, the left front tire starts to "chirp" indicating to me that the tire is at impending lockup. The right side not so much. The caliper on the RF appears to have been replaced in the no too distant past. In the garage with the master cylinder cover off, I can pump up the brakes somewhat and when I release the pedal, I can see fluid gushing back into the master cylinder. That strikes me as a bit odd. So, those are the facts at present. My current plan is to replace all of the rubber brake hoses with stainless steel braided lines, remove all of the bleeders and put Teflon tape on the threads, reinstall and bleed like crazy again. Any suggestions or ideas of what else to check would be greatly appreciated as driving season is here and I'm still in the garage.

 

[bill81vette]

1st Welcome to CAC!!!
How old are your rubber brake lines????
I would NOT put teflon tape on your bleeders..at all!!!!
if you need to,use penetrating spray a couple of time ahead of trying to loosen them.
if you have any more questions just ask..
good luck!

 

[Vettehead Mikey]

The stainless brake lines will not cure the root problem unless the existing lines are bad. As Bill says, do NOT put tape on the bleeder threads. That's not the sealing surface and the tape may make things worse.

The flow-back of fluid into the master at the relief ports after releasing foot pressure is normal.

Did you bench bleed the new master before installing it?

 

[tshera388]

The results of my weekends labors were pretty good. The teflon tape was to prevent air from leaking around the threads of the bleeder while bleeding with an airlock as I am a one man operation in the garage. I took care not to cover the port in the bleeder. I have no idea how old the rubber brake lines were. The rear ones were fairly stiff and a tad brittle. The front hoses were fairly soft and pliable. I did bench bleed the MC before installation. It is amazing how much fluid those things hold. After replacing the lines and taping the bleeders, I pushed another quart of fluid through the system. On the left rear, I observed quit a bit of air being pushed out of the bleeder. Hopefully, this was the demon. There is still some pedal travel, but now when the brakes bite, they bite pretty hard. At least I got to take it for a drive this week. I am still considering pressure bleeding, and also ceramic pads. I have the ceramic pads on my Monte Carlo and they are VERY effective. More later, I am sure. Thanks guys, for your input. It was and continues to be greatly appreciated.

 

[Hib Halverson]

First...on the bleeders...know that they do not seal with their threads. The sealing is accomplished by the tapered surface at the very end of the bleeder and the tapered surface at the bottom of the threaded hole in the caliper.

Take the tape off the threads. If particles of the tape break loose and enter the area below the bleeder they may either plug up the passage the fluid takes during bleeding or, if enough of the tape lodges in the conical sealing surfaces it might actually cause a leak.

It sounds like in your last weekend's session you may have gotten all or most of the air out.

As you say, roadtest for a week then bleed the system again. If you, again, get air out of one or more calipers assume you have a leaking caliper. If they are original calipers, I'd replace them with stainless steel sleeved calipers.

If they are original calipers, you have one or more leaking you also can try a caliper overhaul but, once you have the calipers stripped down, if you find any piston bores rusted or pitted, they cannot be effectively repaired with a hone you must either replace them or have the originals sleeved.

 

[tshera388]

Update on the brake problem. First, Mr. Halverson, I am familiar with how the bleeders work. My problem is, is that in a one man operation air can be pulled around the threads when the bleeder is open compromising the bleeding process. I have used this method in other project cars (mostly MC's) with good results. For the record, I use Chesterton Gold End tape which is a very high quality tape. I honestly do appreciate your concern. Please do not think I am taking your suggestions lightly.

After a couple of drives, I started to lose pedal again accompanied by the annoying "juke" to the left when I hit the brakes. When I got back into the garage tonite, I checked the fluid and found that the front chamber was low. I then jacked up the front of the car and pulled the right front tire. When I depressed the outboard pad, I heard the sickening squishy sound of fluid and air. It would appear that the outboard piston seals are shot (bad caliper as you suspected Hib) Strangely enough, this is the newest caliper on the car. So now my next move is to buy a pair of sleeved calipers and install them and bleed like crazy again. Perhaps Y'all should buy stock in brake fluid.

As always thanks to all who have offered suggestions.

 

[Hib Halverson]

Glad you found the problem.

Unfortunately, the 65-82 disc brake system is prone to air ingestion due to rotor runout and, in the rear, rear bearing play or axle flange run out.

Also, the 65-83 disc brake system with unsleeved calipers is prone to caliper bore corrosion due to moisture in brake fluid attacking the cast iron of the caliper bores.

Those two characteristics make caliper leakage problems common.

Best bet is, if you've found one or more calipers leaking, is to replace all of them with stainless sleeved calipers. CAC Sponsor, Zip Products, sells them.

That said, the stainless sleeving only addresses the corrosion issue. It does not solve any problems with rotor run out. Generally, if runout is greater than .008 to .010-in., you'll have problems with air ingestion. In the rear, rotor runout can be caused by the rotor itself and/or excessive rear bearing play and/or (if the rotor has previously been removed from the axle) the plane of the rotor and the plane of the axle flange not being parallel.

Lastly, it's important that everyone reading understands the bleeding process.

The only part of the bleeding process that is compromised if the bleeder is loose in its threads is the visual part, ie: air bubbles you see may not actually have come from the system ahead of the bleeder fitting.

It is true that air can be drawn from the atmosphere, past the threads, into the flow of fluid out of the bleeder and into your bleeding container, but, as long as there is any flow out of the bleeder, ie: the pressure at the conical bleeder seat is higher than atmospheric, that air which has been sucked past the threads will not be ingested. It goes right out with the brake fluid.

If you're using a pressure bleeder, you don't have to worry about this.

If you're manually bleeding, you just have to remember to close the bleeder while your assistant is still pushing the pedal down. Do not wait for him/her to bottom the pedal before closing the bleeder.

If you're attempting to vacuum bleed a 65-82 system–stop. Vacuum bleeding cannot be used on 65-82 brakes because it's too difficult to do it in a manner which prevents air ingestion past the primary brake pistons seals when the vacuum is applied.

There is additional information on 65-82 brake service here on the CAC at:
Corvette Action Center | Big Block From Hell Series - Part 12
Corvette Action Center | Big Block From Hell Series - Part 13

 

[tshera388]

Update: I replaced the front calipers and bled the system like crazy. The brakes were pretty good, but still not what I was wanting. Tonite I went to the garage to back the car out to do some cleaning. When I stepped on the brakes, they were rock solid. WooHoo! I checked the master cylinder, and the fluid level had dropped about a quarter of an inch. There was no fluid present on the floor, rotors, or rims. I topped off the fluid and went for a test drive just to see. The brakes performed flawlessly. I'm not sure what happened overnite, maybe the brake fairy came. If anybody has any idea what might have occurred let me know. In the meantime I'll take it. As always, thanks for your support.