67 Correct Master Cylinder Replacement

[Kirk M.]

Hey Guys,

Got a properly numbered master cylinder rebuilt and took the power booster off my car this winter and had it cleaned up and re-dichromated "gold". Everything looks great. Now comes the hard part, putting it all back together. Right now the old MC is still on the car, just disconnected from the power booster. So, here are my questions:

1. The brake fluid in the system now is red. The instructions for the rebuilt MC say not to use silicone brake fluid. Is there a way I can tell if mine is currently silicone?

2. What exactly do I need to do to unhook the old MC and re-attach the new one? Will I still need to bleed brake lines in the end? Do I need to "prime" the "new" MC?

I was kinda hoping I could just unhook the old MC and attach the two lines to the new, fill the bowl and be off, but something tells me it won't be that easy. Any help or comments would be appreciated. Thanks.

[David C.]

Re: 67 Correct Master Cylinder Replacement

Hey Guys,

Got a properly numbered master cylinder rebuilt and took the power booster off my car this winter and had it cleaned up and re-dichromated "gold". Everything looks great. Now comes the hard part, putting it all back together. Right now the old MC is still on the car, just disconnected from the power booster. So, here are my questions:

1. The brake fluid in the system now is red. The instructions for the rebuilt MC say not to use silicone brake fluid. Is there a way I can tell if mine is currently silicone?

2. What exactly do I need to do to unhook the old MC and re-attach the new one? Will I still need to bleed brake lines in the end? Do I need to "prime" the "new" MC?

I was kinda hoping I could just unhook the old MC and attach the two lines to the new, fill the bowl and be off, but something tells me it won't be that easy. Any help or comments would be appreciated. Thanks.

Kirk,

I do not know about using silicon in your MC of choice, but this would be a good time to replace the fluid in the entire system. I would use silicon just because of it's better performance in not picking up moisture, but you might want to double check with the supplier.

You should bleed the entire system - this will yield the best results. You should "bench bleed" the MC before installation. This consists of using plastic plugs with rubber hose attachment nipple at the line connections- should be available at a good parts store - try the brand Lisle (and will probably have instructions). Clamp the MC in a vice, install plugs and hoses and route to the MC reservoir, fill with fluid and pump the piston until no air is returned to the reservoir.

You should jack the vehicle all four sides and crack the bleeders at all 4 wheels to let the old fluid drain out with the lines at the MC disconnected (overnight drain if not pumping will work). You can also pump them out with a bleeder pump or vacuum pump. Close off bleeders and install new MC. Bleed the farthest wheel from the MC first, then next farthest and so on. Best to have 2 people even if you are using a bleeder pump - someone to keep filing the reservoir. If you run the reservoir dry you will pull air into the line and maybe even screw up your bench bleed.

After the fourth wheel is done, I would do all 4 again starting at the farthest wheel and repeating to the next farthest.

You are correct in noting that it will not be a simple bolt in and "go" - well, you might "go" just not stop.

Hope this helps,

Dave

[Joe L.]

Re: 67 Correct Master Cylinder Replacement

Hey Guys,

Got a properly numbered master cylinder rebuilt and took the power booster off my car this winter and had it cleaned up and re-dichromated "gold". Everything looks great. Now comes the hard part, putting it all back together. Right now the old MC is still on the car, just disconnected from the power booster. So, here are my questions:

1. The brake fluid in the system now is red. The instructions for the rebuilt MC say not to use silicone brake fluid. Is there a way I can tell if mine is currently silicone?

2. What exactly do I need to do to unhook the old MC and re-attach the new one? Will I still need to bleed brake lines in the end? Do I need to "prime" the "new" MC?

I was kinda hoping I could just unhook the old MC and attach the two lines to the new, fill the bowl and be off, but something tells me it won't be that easy. Any help or comments would be appreciated. Thanks.

Kirk-----

Silicone brake fluid is usually dyed purple and "fades" to yellow in time. However, to confirm what brake fluid you have, remove some and place it in a clear glass vial or jar. Add about an equal amount of water to the jar. Shake it up and let it sit for awhile. If the fluid appears homogeneous after sitting, the fluid is glycol ester-based brake fluid (DOT 3, 4, or 5.5). If there is a distinct phase separation (i.e. like oil and water), then it's silicone fluid.

To remove the old master cylinder you remove the two brake lines, then remove the two nuts that retain it to the booster. Off it comes!

You definitely need to bench bleed ("prime") the master cylinder before installation. No doubt about that, at all. You'll probably need to bleed at the wheels, too. In fact, it's also a good way to renew the brake fluid in the car. You flush out the old fluid with the new as you bleed each caliper.

[Ray G.]

Re: 67 Correct Master Cylinder Replacement

Hello;
Echo what Dave and Joe Said.
Why master cylinder rebuilders recommend not using Silicone fluid may be commented on by someone else, maybe Duke Knows.
We have used it for 30 years with excellent results. None of the negatives of standard brake fluid, like attracting moisture and removing paint.
Determining what fluid is currently used, as Joe wrote, is crucial.
Silicone brake fluid containers read that it will work if mixed with standard fluid, but I think you still have the negatives of standard fluid.
If you bleed silicone into calipers w/ standard fluid w/out draining them you get a slurry mess inside.
Have had success in removing the calipers & hanging them entry end down in a five gallon pail for a day or two will pushing the pistons back in with the back pads. Try not to cock the pistons while doing this, think it may be hard on the seals.
While the calipers are off push denatured alcohol through the brake lines and dry w/ compressed air. Make sure the compressed air is dry if you live in a moist climate.
Bleed according to Service manual.
Hope this helps.
Ray

[Dario C.]

Re: 67 Correct Master Cylinder Replacement

Hi everybody,
Sorry to interupt. I'm new here and just getting into the resto on my car. Have many things to ask but, can I ask if its best to use a vacuum bleeder or pressure bleeder for the brakes?
Right now I only have to replace the seal on one caliper.
Thanks

PS: I have always used silicone fluid on my Harleys which went well. And I have a buddy with several cars (C1's, C2's and he switches each one).

[Kirk M.]

Re: 67 Correct Master Cylinder Replacement

White Post Restorations, the company who did my master cylinder, clearly states NOT to use silicone as there have been several reported failures of their MC when used. So, based upon that, I will not be using silicone. Guess you can check with them to find out why, or if someone from that group is a member, please post.

[Joe L.]

Re: 67 Correct Master Cylinder Replacement

White Post Restorations, the company who did my master cylinder, clearly states NOT to use silicone as there have been several reported failures of their MC when used. So, based upon that, I will not be using silicone. Guess you can check with them to find out why, or if someone from that group is a member, please post.

Dave -----


I've heard of this White Post warning regarding using silicone fluid with their rebuilt master cylinders. To be honest, it gives me great pause regarding the use of a White Post sleeved master cylinder. If there's something about their sleeving process that makes the master cylinder incompatible with silicone fluid, then there's something wrong with their sleeving process. In other words, if one can use a new master cylinder with silicone fluid and one can use a stainless steel sleeved master cylinder with silicone fluid, then why can't one use a White Post brass sleeved master cylinder with silicone fluid?

[Kirk M.]

Re: 67 Correct Master Cylinder Replacement

Good question. Maybe someone else has been through this before and knows the answer. Unfortunately, for me it's too late and I'll only be using non-silicone brake fluid. Someone help us out here!

[John H.]

Re: 67 Correct Master Cylinder Replacement

Good question. Maybe someone else has been through this before and knows the answer. Unfortunately, for me it's too late and I'll only be using non-silicone brake fluid. Someone help us out here!

Kirk -

Before you do anything, do the fluid-and-water-in-a-jar trick and find out for SURE what kind of fluid you have in the system now.

[Joe L.]

Re: 67 Correct Master Cylinder Replacement

Good question. Maybe someone else has been through this before and knows the answer. Unfortunately, for me it's too late and I'll only be using non-silicone brake fluid. Someone help us out here!

Kirk----

Make sure you NEVER let even a drop of that glycol-based fluid get on your paint. If you do, you'll wish that you had thrown that White Post master cylinder in the scrap barrel and obtained another one that was compatible with silicone brake fluid. Guaranteed!

[John H.]

Re: 67 Correct Master Cylinder Replacement

Dave -----


I've heard of this White Post warning regarding using silicone fluid with their rebuilt master cylinders. To be honest, it gives me great pause regarding the use of a White Post sleeved master cylinder. If there's something about their sleeving process that makes the master cylinder incompatible with silicone fluid, then there's something wrong with their sleeving process. In other words, if one can use a new master cylinder with silicone fluid and one can use a stainless steel sleeved master cylinder with silicone fluid, then why can't one use a White Post brass sleeved master cylinder with silicone fluid?

Joe -

I have no idea why Billy still provides that silly warning - maybe the only seals he can find for old Packards and Duesenbergs aren't compatible with silicone fluid. He's done an armload of stuff for me over the last 30 years, including some priceless/irreplaceable early 60's Ferrari aluminum wheel and clutch cylinders, and all have been top-notch. Will have to ask him one of these days what his beef is with silicone fluid.

[Kirk M.]

Re: 67 Correct Master Cylinder Replacement

Here is what his warning states:

"Please note: We do not recommend the use of silicone brake fluid. Some usage has been successful and many have had trouble. This fluid will swell some grades of rubber causing your brakes to lock. It will soften and jell some rubber causing your cylinders to leak. We suggest that you use the kind of fluid recommended for your car and change it in a few years."

So, my question is exactly what is the "rubber" he is talking about? The rubber seal in the MC?

[Terry M.]

Re: 67 Correct Master Cylinder Replacement

I have had DOT 5 fluid in my 1970 for over 20 years with no ill effects. I have flushed and refilled with new DOT 5 once in that time, and will likely do that again. The calipers were sleeved by a place in Texas, but I cannot tell you the name without some searching. They did offer a service to only do the sleeves, and I assembled the rest of the calipers using the pistons and seals of my choice. I did use Delco service parts to rebuild the master cylinder and calipers.

It may be all we have here is a testimony for the use of Delco parts.

[Joe L.]

Re: 67 Correct Master Cylinder Replacement

Here is what his warning states:

"Please note: We do not recommend the use of silicone brake fluid. Some usage has been successful and many have had trouble. This fluid will swell some grades of rubber causing your brakes to lock. It will soften and jell some rubber causing your cylinders to leak. We suggest that you use the kind of fluid recommended for your car and change it in a few years."

So, my question is exactly what is the "rubber" he is talking about? The rubber seal in the MC?

Kirk-----


The "rubber" they are undoubtedly referring to are the seals in the master cylinder. If one were using old, antique, NOS seals (which some may have to do because the seals are not currently manufactured and haven't been in years), then I could understand why there might be a problem. Such seals are likely made of material which is not compatible with silicone brake fluid.

Corvette master cylinders are usually rebuilt with seals manufactured from modern seal materials. I believe that any DOT-approved brake parts have to be compatible with DOT-approved brake fluid formulations. Of course, many of the older, antique seals I am talking about were manufactured before there even was a DOT.

[Joe R.]

Re: 67 Correct Master Cylinder Replacement

Good question. Maybe someone else has been through this before and knows the answer. Unfortunately, for me it's too late and I'll only be using non-silicone brake fluid. Someone help us out here!

Hi Kirk:

I'm glad to see that you got that MC rebuilt and you are putting it on the car. I read this whole thread and I think others have already told you what you need to know, but I want to reinforce a few things:

1) Billy's warning about silicone fluid is probably overdone. If your system was all freshly rebuilt and empty, you could probably use silicone fluid and be fine. However, your system still has a lot of the old fluid (presumably of the glycol-ester type) in the calipers, and you can't get all that out without removing the calipers. Many people have reported problems mixing silicone fluid with glycol-ester fluid, so that alone is a good reason not to use silicone fluid if your system has glycol-ester fluid in it now.

2) By all means, get a bench bleeding kit from an auto parts store such as NAPA and bleed the MC on the bench, being sure to push the piston ALL the way to the end of its travel. You need to get 100% of the air out of the MC before you install it (don't ask me how I know this).

3) Glycol-ester brake fluid dissolves paint rather well. Install plastic shielding over everything that the fluid might get on.

4) Once there is fluid in the MC, the fluid will want to slowly run out of the two outlets. Be prepared for this and plan your assembly sequence so that you can get the two brake lines installed quickly. It would be best to attempt a dry run before you fill the MC, to make sure that you can easily get the brake line fittings started.

5) Once the MC is filled and bled, don't let the fluid level go all the way down so that air gets in the filler holes in the bottom of the resevoir. That will defeat your bench bleed.

6) If you check the archives, you will find LOTS of discussion on bleeding techniques. Pressure bleeding is nice, but the commonly available kit (can't remember the name) does not seal well to the MC and needs to be augmented with a large C-clamp. John Hinckley once posted a nice photo of the C-clamp arrangement. I believe that if your MC has been properly bench bled, you can use the so-called "gravity bleed" system and get good results.


I hope the above helps to reinforce what the others have been saying. Good luck getting this done!