Dual chamber Master Cylinder conversion

[Flathead Fever]

I worked on a fleet of 400 vehicles, a lot of those vehicles still had single master cylinders. Not once did I ever see a "single" brake system completely fail. With the exception of when a snow chain cross-link broke and sheared the metal brake line right off the wheel cylinders. That happened several times. That was the drivers fault for letting the cross-links wear too thin. We had bags and bags of new cross-links. All they had to do was ask and I would crimp-on new set of cross-links for them.

Back then, those phone company vehicles were being serviced every three-months. Part of that inspection was to put them up on a hoist and pull the drums and inspect the brakes. We pulled back the rubber dust boots on the wheel cylinders to see if they were beginning to leak. If we saw a little moisture we ignored it. If there was actual liquid behind the boot we rebuilt the cylinder and the one on the opposite side. We had an plastic box assortment of wheel cylinder cup sizes. We ran a ball hone through the cylinders. Wiped the bore clean with brake fluid and a paper towel until nothing showed up on the white paper. We replaced just the cups unless the outer boots were cracked then we used a kit with the piston, cups and the boots. A kit was half the price of a new wheel cylinder and just replacing the cups was probably less than a dollar a wheel cylinder. A lot of time you could leave the wheel cylinder on the vehicle and still run a hone through them. That way you did not need to disturb the metal brake line. On the mountain trucks the nuts on the brake lines would rust to the tubing and the tubing would twist with the nut. On those it was better not to mess with them so you didn't end-up making new metal lines. We rebuilt the wheel cylinders for year's, until we got bought out and the new corporation decided it was too big of a liability. We still did it if it kept a vehicle in service. You just didn't write it in the record's. I never saw a wheel cylinder completely fail.

We looked at the back of the master cylinders on each service to see if they were beginning to leak. You would see fluid running down the inside of the firewall if it was leaking. If the master cylinder was bolted to a vacuum brake booster then you could not see the back of the master cylinder. On those you pulled the vacuum line on the booster and stuck a small piece of rubber tubing down in there to make sure there was no brake fluid in the bottom of the booster, from a leaking master cylinder. You use a rubber hose because something like a welding rod could poke a hole in the booster's rubber diaphragm. If something is going to fail in a single brake system its going to be the master cylinder. Not because a seal completely blew-out but because of neglect. You could have a slow leak and the reservoir could run out of fluid. If you check your fluid level and its low you need to look at the back of the master cylinder for leakage. If its not leaking there and has lost a lot of fluid from a wheel cylinder you will see brake fluid on the brake backing plate. It might even being running down the tire if the car has been sitting a long time. The other thing that can happen to a master cylinder is dirt can block the little ports that allow the fluid to flow from the reservoir to the piston. You will press on the pedal and you won't have any. You step on it again and you have a normal pedal. If your master cylinder looks like mud inside this might happen to you. This only ever happened to me once and it was on my first car, a '65 Mustang fastback that was twelve-year's old. So was the brake fluid.

We also checked the rubber brake lines for cracks. They usually outlast the vehicle. We did have a recall on some of larger C-50 Chevy trucks for the right brake hose failing. They used the same hose on the left and on the right so what think a minute, what could possibly cause just the right hose to fail? The battery box was mounted right above the hose and the acid would drip down onto the crimped-on metal end of the brake hose and it would rust away. Then the rubber hose would blow-off the fitting. That took years to happen. I never saw a brake hose fail in the 30-year's I was there.

I have no problem trusting a single brake system that I have inspected. They are perfectly safe if maintained properly. Unless something rips off a brake hose or a steel line the system will not completely fail. If something does hit a steel line it usually crushes it but it doesn't tear. The most important thing is you don't have a dashboard warning light to tell you that your low on brake fluid. You have to check it once in awhile. I always glance at my tires when I'm walking up to my vehicle to make sure they are not going flat. At the same time I look at the bottom of brake backing plates to make sure they are not wet. If its been a long time since I have driven the vehicle I check the brake fluid level and the pedal travel.

I went to a Bendix brake training class. The instructor said that if you flushed out your brake fluid every two-year's the hydraulic parts like the master cylinder and wheel cylinders would last forever. Its the moisture that gets in the fluid that does the damage. Brake fluid is hydroscopic. That means it absorbs water at about 2% a year. This is funny, word search does not recognize "hydroscopic" its trying to change it to "hydroponic" There must be more marijuana growers than brake mechanics. Even though your brake system is sealed it expands and contracts with temperature change. This can draw moisture in past the seals. The most neglected system on a vehicle is the most important, the hydraulic portion of the brakes. Nobody ever changes their brake fluid? If its brown that is rust in your fluid. It can only rust if there is water in your brake fluid. Once it starts rusting its impossible to stop it. You can put new fluid in and it will just turn brown again. If the fluid is black, that is the your rubber cups going deteriorating. Its a hint that you should rebuild all the cylinders.

At the very least, on your daily drivers change the fluid every time you do a brake job. Your antique cars should still have their fluid changed every two year's but that's probably not going to happen, even in my own garage.

Next item. Built into your drum brake, master cylinder, is a residual check valve. It allows the brake fluid to return to the reservoir when you release the brake pedal but then it closes off leaving just a little pressure still in the system. Its not enough pressure to keep the wheel cylinder pistons from returning but its enough to push out the lips on the rubber cups, in the wheel cylinders, against the cylinder's bore and create a seal. This helps prevent the wheel cylinders from leaking. If the vehicle sits for a longtime that pressure is going to bleed off. If you have cars you don't drive at least get in them and stomp on the brakes a few times every once and a while. That will put the pressure back on the wheel cylinder cups.

Keep your brake fluid clean. Check your fluid level often. Make sure when you do check it that when you put the lid back on the master cylinder that the gasket is in place. If its not, your fluid could slosh out of the reservoir. This happens! There is nothing wrong with a single brake system if its maintained properly. It will not completely fail, except in old black and white movies.