Master Cylinder suddenly empty

[Frederick W.]

Went out to fire up my 68 'vette today and wake it from it's winter slumber. As I went to back it out of the garage I noticed the brake pedal went to the floor. Popped the cover off the master cylinder and the front reservoir was completely empty (rear was fine).
I crawled under and looked at both brake calipers and didn't see any obvious leaks.

So it seems there must be a leak somewhere. What is the most likely culprit, and what is the best sequence of analysis to determine the source?

Thanks
Fred
Tulsa

[Joe L.]

Re: Master Cylinder suddenly empty

Went out to fire up my 68 'vette today and wake it from it's winter slumber. As I went to back it out of the garage I noticed the brake pedal went to the floor. Popped the cover off the master cylinder and the front reservoir was completely empty (rear was fine).
I crawled under and looked at both brake calipers and didn't see any obvious leaks.

So it seems there must be a leak somewhere. What is the most likely culprit, and what is the best sequence of analysis to determine the source?

Thanks
Fred
Tulsa

Fred-------


By far the most likely is a leak from the calipers. Dot 3 fluid is fairly volatile and it's very possible that the leakage evaporated over time.

[Rich G.]

Re: Master Cylinder suddenly empty

I had the same experience last year with my 68, power brakes. The front chamber was empty. I added fluid and it's been fine ever since. I have to admit I neglected to check it for a long time before this happened so I'm not sure what happened. This car doesn't get used much.

I guess if you fill it and look closely for leaks you will see something. I couldn't find any leaks on mine. As I say, it has remained full for a year.

Rich

[Edward J.]

Re: Master Cylinder suddenly empty

Fred, There must be a small leak somewhere, likely a caliper leak somewhere, they will generally be dripping of the bottom lower of the caliper. i would advise putting the car on a lift for a closer inspection if you can see a leak anywhere. sometimes the master cylinder will leak and may go into the internals of the brake booster, to inspect this just back out the two nuts and pull away and look for wetness at rear of cylinder.

[Frederick W.]

Re: Master Cylinder suddenly empty

Thanks for the suggestions, guys.

I found this on the Vette Brakes and products website.
They say brake fluid doesn't evaporate. ?.

I'll get it on my lift as soon as I get a chance and move the other car...

[Edward J.]

Re: Master Cylinder suddenly empty

Fred, I have never heard of brake fluid evaporating but will easily wash away with rain water when driven. maybe a humid climate to have some effect on a very small leak..

[Joe M.]

Re: Master Cylinder suddenly empty

If you have power brakes the fluid may very well be in the brake booster can.

[Joe L.]

Re: Master Cylinder suddenly empty

Thanks for the suggestions, guys.

I found this on the Vette Brakes and products website.
They say brake fluid doesn't evaporate. ?.

I'll get it on my lift as soon as I get a chance and move the other car...

Frederick------


I think they are wrong. In my experience it does evaporate. It will definitely not evaporate from a closed and sealed system. However, in my experience it does evaporate when it's not contained.

[Jim T.]

Re: Master Cylinder suddenly empty

If you have power brakes the fluid may very well be in the brake booster can.

Joe if the front cavity for brake fluid in the master cylinder is empty and the rear cavity is full of brake fluid, how does the front cavity of brake fluid transition into the power brake unit?

[Rich G.]

Re: Master Cylinder suddenly empty

Just had the 68 out and checked the fluid and it is down a bit in the front, so looks like I'll be looking for a leak, too.

Rich

[Duke W.]

Re: Master Cylinder suddenly empty

All liquids have a vapor pressure, so they will all (eventually) evaporate, but brake fluid has very low vapor pressure, so leaking brake fluid will usually leave a stain for a long time. Get the front end up in the air, remove the wheels and do a close inspection. Evidence of brake fluid leakage should be evident unless the car has been driven in a lot of rain. Conventional DOT 3 or 4 brake fluid is water soluble.

The other thing to consider is that disk brake system reservoir level slowly drops as the pads wear and fluid is displaced to the caliper, and for this reason disk brake fluid reservoirs are always large to provide extra fluid to fill the void produced as the pads wear, but this level drop should very very slow as miles accumulate.

This fluid displacement does not occur on drum brake systems because the adjustment for shoe wear is mechanical at each corner. That's why four-wheel drum master cylinder capacity is usually less than total (one or two reservoirs) capacity of a single or dual ciurcuit four-wheel disk system. On disk/drum systems the reservoir that serves the disk half of the system is usually much larger than the reservoir that serves the drum half of the system.

Brake fluid should be checked at least annually. If you store the car in winter, check it prior to putting the car away and again when you bring it back into service in the spring as part of your normal pre and post storage check list. The "full" level is usually stated as 1/4" below the top of the reservoir, but see your service manual.

Duke

[Terry M.]

Re: Master Cylinder suddenly empty

We all seem to be thinking that the front master cylinder reservoir supplies the front braking system. I am not so sure that is true, but it has been way too long since I bled my Corvette brakes, and I have way too many other 4-wheel disk brake vehicles to service -- so I may be confused. Please verify which end of the car which reservoir supplies.

[Jim T.]

Re: Master Cylinder suddenly empty

A few years ago my front brake calipers were leaking on my 1968 and I replaced the seals. Only the front reservoir was low on brake fluid and filled. Gravity bled the calipers.

[John H.]

Re: Master Cylinder suddenly empty

We all seem to be thinking that the front master cylinder reservoir supplies the front braking system. I am not so sure that is true, but it has been way too long since I bled my Corvette brakes, and I have way too many other 4-wheel disk brake vehicles to service -- so I may be confused. Please verify which end of the car which reservoir supplies.

Terry -

On Corvettes, the front feeds the front and the rear feeds the rear. Always.

[Terry M.]

Re: Master Cylinder suddenly empty

Terry -

On Corvettes, the front feeds the front and the rear feeds the rear. Always.

Thanks John. Too many GM cars in the stable. It must be the Caprices that are backwards.