Brake Master Cylinder?

[KulasO313]

Hey everyone,

I have an issue with my brakes at the moment. I dont know when the last time my 67 corvair was driven as I have yet to do so, but for some reason the brake pedal does not return to the top after pressing it. All 4 wheel cylinders seem to be working because when I push the brake, the wheels freeze up, and when I release, they free up. Therefore I believe the wheel cylinders are ok. Is there supposed to be some sort of return spring on the brake pedal on these cars? Or does it sound like I need a new master cylinder? Oddly enough, it looks like an aftermarket master cylinder is already in the car. Thanks for any ifo

[terribleted]

The only return spring in inside the master itself. Sounds like either the pedal is way out adjustment or the master cylinder is sticking.

[bbodie52]

Brake fluid does not compress, but air does. The brake fluid serves as a fluid coupling, or linkage, between the brake pedal and the brake shoes. There are also springs attached to each brake shoe. When you press on the brake pedal the fluid linkage between the master cylinder and the wheel cylinders moves in accordance with your foot pressure -- moving the brake shoes into contact with the brake drums. When your foot releases pressure from the brake pedal, the springs in each wheel brake assembly pull the shoes away from the drums, and the motion is transmitted from the four wheel cylinders back to the master cylinder -- returning the brake pedal to its original position.

If there is some air in the brake lines and brake cylinders, the fluid coupling becomes "spongy", as the air compresses somewhat before pressure is applied directly to the brake components at the wheels. The reciprocal is also true, in that the motion of the springs returning the shoes to their original positions may not translate directly to moving the brake pedal completely back to its original position.

Since brake fluid absorbs moisture (water) from the air (unless it is silicone brake fluid), the presence of air in the lines may also indicate the possibility of brake fluid that may be excessively contaminated with water. Since water has a lower boiling point than brake fluid, excessive water content can also mean a lower boiling point as the brakes become hot in use, which translates into premature brake fade as the fluid boils.

If your brake pedal travels excessively and feels spongy, I would recommend flushing the brake system... bleeding it enough from each wheel cylinder to completely remove the presence of any air in the hydraulic system, and also removing all of the old fluid and replacing it with fresh fluid. Clark's Corvair Parts recommends replacing the old fluid with DOT 5 Silicone brake fluid, which does not absorb water from the air. This eliminates the recommended periodic flushing of the Corvair brake system every few years, and reduces rust and corrosion possibilities in the master cylinder, wheel cylinders, and steel brake lines. The condition of the flexible brake lines at each wheel should also be inspected.

A brake component inspection, fluid flush and fluid replacement, and bleeding all air from the system amounts to a brake system tune-up. Your description of the pedal failing to fully return may be a sign of a need for such a brake system "tune-up".

[bobs1962]

I just replaced all 4 brake cylinders, hoses and (almost all) the lines ((I'm still working on the long, middle one). It sounds like DOT 5 with silicon is what I should use. Do you agree?

[bbodie52]

The Clark's Corvair Parts online catalog page recommendation would seem to support the switch to DOT 5 silicone brake fluid. Their reputation and decades of Corvair experience would certainly give credibility to their recommendation! I am making the switch to DOT 5 on my 1966 Corvair Corsa.

http://www.corvair.com/user-cgi/catalog ... N&page=144


A price comparison between DOT 5 Silicone and DOT 4 Brake Fluid does reveal a higher price for DOT 5 brake fluid, but the advantages of using DOT 5 brake fluid would seem to outweigh the cost disadvantage, especially in the long-term since DOT 5 fluid would not have to be replaced periodically to avoid water contamination buildup.

http://www.amazon.com/Gunk-M4032-Silico ... luid+dot+5

Gunk M4032/6 DOT 5 Silicone Brake Fluid - 32 fl. oz.
by Gunk
List Price: $29.99
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Ships from and sold by Amazon.com.
Meets or exceeds FMVSS No. 116
Does not absorb water
Rated at plus 500 degree Fahrenheit
Insures long term braking performance under extremely high and low operating temperatures

Gunk M4532 DOT 4 Super Heavy Duty Brake Fluid - 32 fl. oz.
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[Sakubyrd]

Sorry to hijack the post, but my son's 61 900 was pulling to the right when braking. Took it to the local Les Schwabs and they said the brake system may be "contaminated". Not sure what that means. According to them, if it is "contaminated" it will cost 1-2 k to fix. I think they're full of something, but could they be right?

[bbodie52]

"Contaminated" could be simply air in the lines, that can be corrected by bleeding the air from the system. If the fluid is old there could also be water contamination in the brake fluid, which can cause premature fading (the water contamination lowers the boiling point). Flushing the old fluid out and replacing it with fresh fluid, or upgrading it with DOT 5 Silicone brake fluid at the same time can rid you of the contaminated fluid.

http://en.wikipedia.org/wiki/Brake_fluid

Brake fluid is subjected to very high temperatures, especially in the wheel cylinders of drum brakes and disk brake calipers. It must have a high boiling point to avoid vaporizing in the lines. This vaporization is a problem because vapor is highly compressible relative to liquid, and therefore negates the hydraulic transfer of braking force.

Quality standards refer to a brake fluid's "dry" and "wet" boiling points. Wet boiling point, which is usually much lower (although above most normal service temperatures), refers to the fluid's boiling point after absorbing a certain amount of moisture. This is several (single digit) percent, varying from formulation to formulation. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic(water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5-based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life, although at the cost of potential phase separation/water pooling and freezing/boiling in the system over time - the main reason single phase hygroscopic fluids are used.

In the worst case, extended exposure to brake fluid that contains a high percentage of water contamination could possibly have caused rust and corrosion in the master cylinder, wheel cylinders, and the steel brake lines. If it is bad enough it could cause the pistons in the cylinders to stick and to apply pressure unevenly to the brake shoes, or to fail to release pressure if a piston sticks and will not return. A rusted-out steel brake line can burst. The possibility of losing all brake pressure because of a burst steel line or a burst flexible line was the reason that dual master cylinders were introduced to all cars in 1967. Clark's sells dual master cylinder conversion kits, but unfortunately they only fit 1962-1966 Corvairs.

I would suggest flushing your system and replacing the fluid with DOT 5 Silicone brake fluid. In the process you would also be removing any air in the system. The process is not complicated and can be performed easily with two people (one to work the brake pedal and refill the master cylinder, and the other to climb under the car and open the wheel cylinder bleed valves to drain the air and old fluid). I have attached a copy of the appropriate section of the 1961 Corvair Shop Manual to guide you. If you have any questions about the procedure, please ask.

This video describes the basic procedure. There are numerous videos on YouTube describing this. I selected one video, but there are others. Most describe the procedure on modern cars with dual master cylinders, disc brakes, etc. But it is not that much different on the old Corvair... just look over the instructions in the shop manual.



If you still find the brakes pull to one side after you have flushed the system, one or more wheel cylinders could be corroded and may have to be rebuilt or replaced. But if you do the work yourself you will learn from the experience, add a few tools to your tool inventory, and probably enjoy the learning experience if you have never performed such work on your car before.

The following Corvair Forum link can provide you with a list of useful websites that should be helpful as a Corvair owner...

Common and Useful Corvair Websites

Corvair Forum viewtopic.php?f=225&t=6007

http://www.amazon.com/gp/product/B00200 ... duct_title
Gunk M4032/6 DOT 5 Silicone Brake Fluid - 32 fl. oz.
by Gunk
2 customer reviews
List Price: $29.99
Price: $22.28 & FREE Shipping on orders over $35.


http://www.corvair.com/user-cgi/catalog ... N&page=144


Part number C3797: QUART SILICONE BRAKE FLUID

Weight: 3 lbs 0 oz
Catalog Pages(s): 144,R,24,OT,44
Price: $ 29.30