Brake fade question

[bwing]

I haven't had any problems with the brakes since I purchased the car in June, '09. Recently have had minimal brake fade which progressed to the brake pedal going all the way to the floor. I found that the rubber boot inside the master cylinder was "off" a bit and just a little brake fluid escaped. No signs of brake fluid dripping on the garage floor. I realigned the rubber boot but still have slight brake fade when you first depress the brake pedal. How is this so? Do the brakes need bleeding now? ;help

 

[Vettehead Mikey]

That's not really brake 'fade' in the normal use of the word. Fade occurs when the brakes are used repeatedly and approach the point of overheating. The expansion of the brake drum from this heat causes it grow to the point where the shoes can no longer reach it. Very exciting!

Sounds like you have developed a slight leak. Address that first, and bleed the system aftewards.

 

[67l36driver]

You need to install new seals on the piston in your master cylinder. About a one hour job but I could make it last a couple days.

Master cylinder kit is likely less than ten bucks at your flaps.

 

[bwing]

Vettehead Mikey...thanks for your input. I've pulled the wheels and checked the calipers and there is no leak or any sign of the tell-tale leak of brake fluid sprayed on the tires. Also, there is no leak from the master cylinder.

67l36driver...since there are no leaks, this sounds the most logical reason as to the spongey brake pedal feeling. I'll give that a go first. Thanks. FLAPS? Favorite Local Auto Parst Store or Friendly Local Auto Parts Store?

 

[Vettehead Mikey]

I found that the rubber boot inside the master cylinder was "off" a bit and just a little brake fluid escaped.

Also, there is no leak from the master cylinder.

Well, that's confusing. :confused ANY seepage is too much. You could try bleeding, but sort out the master first.

 

[Hib Halverson]

Well, that's confusing. :confused ANY seepage is too much. You could try bleeding, but sort out the master first.

"Vetehead Mikey" is right.

Any leakage out the rear of the master into that book is a problem.

 

[bwing]

Thanks for the input guys. I'm going to look into it today.

 

[bwing]

Well, that's confusing. :confused ANY seepage is too much. You could try bleeding, but sort out the master first.

Worked on the brake problem today. Found that there was minimal leakage from the master cylinder cap. Not enough to drip onto the floor though. No other leaks found. Had a neighbor step on the brake pedal for me and I found the master cylinder cap slightly rising up when the brake pedal was pushed. Looked closer and saw that the end of the clip that attaches to the cap was pulling the tabs down when the brake pedal was depressed causing master cylinder to lose pressure. Had to "roll" the tabs back into place and everything works fine now. It must be a poor design of the cap tabs or the cap I've got is defective.

 

[Vettehead Mikey]

Don't really follow your explanation of what was wrong. The master cylinder reservoir is not sealed nor does it become pressurized at any time, in fact it won't work if it is not vented to atmosphere.

Got a photo?

 

[bwing]

Don't really follow your explanation of what was wrong. The master cylinder reservoir is not sealed nor does it become pressurized at any time, in fact it won't work if it is not vented to atmosphere.

Got a photo?

The clips on the side of the master cylinder were not secure because the tabs on the cap (where my fat finger is pointing...) were slightly pulled down allowing the cap to lift up when the brake pedal was pressed down. It all seems to work fine, now that I've put the tabs back to their correct position.

 

[Hib Halverson]

This is another case of people asking for help but not posting what model year car the question is about. It's a PIA when people fail to do that.

The vehicle info for the first post says the poster owns an 03 and a 96, but the picture posted is from a C2, now that I see those pictures, I can better understand the problem.

The first problem mentioned--a soft, low or receding brake pedal is typical of either a failed master cylinder (internal leakage) or problematic (leaking) brake calipers. You need to assess the condition of the brake caliper piston seals and the master cylinder and determine which component is leaking. If if it's the master, I'd either rebuild it (if you have the tools to hone the bore) or replace it. If you find caliper piston seal leakage (65-67) or wheel cylinder leakage (63-64) replace the calipers with stainless steel sleeved rebuilds or rebuild or replace the wheel cylinders. Additionally, check all fittings and brake hoses for leaks and repair or replace as necessary.

As to the second "problem" which sounds like a small amount of brake fluid getting trapped between the master cylinder reservoir lid and the rubber seal, that's normal. As long as the seal is not torn or has holes in it and as long as it seals against the master cylinder reservoir rim, I'd remove the seal, wipe up the brake fluid, reinstall the seal and put the cap back on.

 

[bwing]

If the problems are 1) a soft or receeding brake pedal and 2) a slight amount of brake fluid dripping of the rubber seal which is inside the master cylinder cap, they are unrelated and one is abnormal but two is normal.

If the pedal is low, soft or receeds under pressure, there's a hydraulic problem somewhere in the system that needs to be addressed. Start by bleeding the brakes per the instructions in the Factory Service Manual for the model year in question.

Boy, you guys aren't giving this retired guy a rest. All seems good with the brakes but I understand your concern so I will bleed the brakes today. Time to break out the FSM.

 

[Hib Halverson]

I've rewritten my earlier answer because I didn't see the photos you posted until later.

 

[bwing]

I've rewritten my earlier answer because I didn't see the photos you posted until later.

My mistake. I should have posted the vehicle year and yes, it's a C2...1966. There are NO leaks anywhere except that minimal dripping that came from the master cylinder cap. All brake calipers have been replaced with new and there is no leakage coming from the piston rubber boots. No leaks from any brake line connection and no cracks/bends in brake lines. Brakes are working fine after I straightened the master cylinder clip tabs and no further leakage from the master cylinder cap. The two tension hold down clips are now tight. Going out to bleed the brakes anyway just to be on the safe side.

 

[Vettehead Mikey]

Brakes are working fine after I straightened the master cylinder clip tabs and no further leakage from the master cylinder cap.

Again, the leak from around the cap seal has no bearing on the low pedal condition you previously described. I wouldn't presume that you've fixed the problem yet.

 

[bwing]

Again, the leak from around the cap seal has no bearing on the low pedal condition you previously described. I wouldn't presume that you've fixed the problem yet.

Ok, you've completely lost me with this one. The pedal fade is on a 1966 w/o pwr brakes. I thought the master cylinder cap had to be completely sealed (inside cap rubber gasket) to the master cylinder. From what you're saying, the brakes will still work with the master cylinder cap not completely secure/flush with the master cylinder. Do I follow you ok? I'm brought up to believe that the cap has to be securely fastened to the master cylinder for you to have holding brake pressure. Am I wrong in this assumption? I have complete brakes now that I made it so the cap is secure, with no leakage. I have no other leaks in the entire brake system and I have no brake fade, soft pedal, now. (I did bleed my brakes a few minutes ago as per Hib's suggestion.) But there was no pedal fade/soft pedal before I bled them. So I can't figure out where my soft pedal/fade was coming from other than the loose fitting cap that allowed just a little fluid and pressure to escape.

 

[Vettehead Mikey]

I'm brought up to believe that the cap has to be securely fastened to the master cylinder for you to have holding brake pressure. Am I wrong in this assumption?

Your assumption is incorrect. Have a look at this cross section (not Corvette but close enough) The cap is vented to the atmosphere as noted. The only section of the M/C that is pressurized is the cylinder in front of the cup when it is pushed far enough by the piston to cover the bypass port. The remainder is at atmospheric.

Not shown in this diagram is a rubber bellows underneath the cap, this ensures that the fluid is not contaminated by air.

 

[bwing]

I can see by this diagram what you are trying to get through to me but still don't understand. Must be the mechanically challenged in me. I see the cap has what looks to be a rubber gasket that is pushed up to seal any air/brake fluid from escaping when the brake pedal is depressed. If that seal weren't there your brake pedal would go to the floor without the back pressure..correct? That's the point I'm trying to make. The cap on my master cylinder has no vent hole. There is a hole in the rubber gasket underneath the cap which I think is the vent but it only has ventilation from whatever air is between the gasket and the cap. Thanks for the help.

 

[Hib Halverson]

Forget that rubber seal for now. It's not a significant issue, as you'll see in a minute.

Let's get back to the brake system's operation.

The use of the term "fade" to describe the brakes' problem might be incorrect. "Brake fade" refers to the car's braking being degraded due to the brake pads and/or brake rotors getting too hot.

The problem you seem to be trying to describe is either a low pedal or a receding pedal. A "receding" pedal is when you put your foot hard on the brake pedal and it slowly goes down to the floor. With the 65-82 brake system, this usually is demonstrative of either leaking brake caliper piston seals or a leak in the master cylinder, either internally, or out the rear of the master.

Is a receding pedal the problem with this car?

Ok, now the "seal" or "boot" on the bottom of the master cylinder reservoir lid. When you remove the seal, you may find a small amount of brake fluid trapped between the top of the seal and the bottom of the lid. That is normal, however, is the rubber seal does not fully seal between the machined flat surface on the rim of the master cylinder reservoir and the cylnder cap, then it needs to be replaced. That said, if the seal is faulty and is allowing some leakage out of the reservoir then a jet of brake fluid shoots up as the master cylinder piston returns after a brake application, that leakage is not related to a problem with a receding brake pedal.

If you do have a problem with low or receding brake pedal, solve that problem first, then replace the master cylinder cap seal.

 

[JohnZ]

I can see by this diagram what you are trying to get through to me but still don't understand. Must be the mechanically challenged in me. I see the cap has what looks to be a rubber gasket that is pushed up to seal any air/brake fluid from escaping when the brake pedal is depressed. If that seal weren't there your brake pedal would go to the floor without the back pressure..correct?

Nope. The only fluid under pressure when you step on the brake is in the bore of the cylinder - as soon as the piston (with the seal cup on the front) passes the bypass hole that connects the bore with the reservoir, pressure builds in the bore and in the brake system (the diagram doesn't show the outlet at the end of the bore). It only takes about .030" of piston movement to move the seal cup past the bypass hole, and at that point the reservoir is completely isolated from the bore, and remains at atmospheric pressure - the reservoir is NEVER under pressure.

Your cap just has a simple convoluted rubber seal to the rim of the reservoir, to prevent fluid leakage and to keep air away from contact with the fluid, and the area between the inside of the cap and the top side of the seal is vented to atmosphere through a tiny hole in the cap.

If the reservoir is full and there are no fluid leaks in the lines or at the calipers and the pedal goes to the floor, the fluid in the master cylinder bore is bypassing the piston seals and the cylinder needs rebuilding.