Corvette Forum Project C4 Part 1: Changing the Master Cylinder

Our new project starts with a thorough once-over before addressing a dangerous leak in the braking system.

Like most American boys, Corvettes were one of my first schoolboy crushes. As a result, getting to write about them for the past three and a half years here at Corvette Forum has been a real privilege.

I’ve spent much of that time singing the praises of the C4 as a fantastic performance bargain. This year, I decided to put my money where my mouth is and buy one of my own. Inspired by two good friends who own C4s, I started hunting for my perfect Corvette.

I found it just a few hours south, over the border into West Virginia. It’s a yellow 1986 with all of the right options — it’s a Z51 package car with Delco Bilstein shocks, the auxiliary radiator fan, engine oil cooler, and the 3.07:1 performance rear axle ratio. It was also ordered with a transparent roof and cloth seats, a unique, but sensible configuration. Like almost all Corvettes built in 1986, it also has the Delco Bose stereo.

While I prefer to shift my own gears, I’m not a big fan of the Doug Nash 4+3 manual that these early cars came with, so the 700R4 automatic is just fine for me. The car’s interior was disassembled, and it was in need of some mechanical work as well. Knowing its restoration would make a great Corvette Forum project, I struck a deal and brought it home.

Initially, I had planned on sending my Corvette to the body shop right away for fresh paint. It may look fine at first glance, but as you can see in the video, looks can be deceiving. Light colors hide many sins, and this bright yellow Corvette was no different.

Tracking Down Leaks

However, before I could bring the car over to my buddy’s paint shop, I had to address a major leak underneath the car. I didn’t want to make a mess of his floor, after all. The issue could be traced to a failed brake master cylinder. Every time the brake pedal was depressed, fluid leaked down the booster onto the ground below. Not only is it messy, it’s unsafe, too. It needed to be fixed right away.

Brake master cylinders for the early ABS-equipped C4s are in short supply, and when you can actually find them in stock, they’re expensive. Thankfully, I had an ace in the hole: my parts car.

Shortly after bringing the ‘86 home, I bought a red 1987 Corvette that was the unfortunate victim of an engine fire with just 34,000 miles on the odometer. I was mainly interested in the mint-condition seats and the rest of the interior, but, as it turns out, there are plenty of parts I’ll be able to use from this car, including the brake master cylinder.

The parts car’s master cylinder was removed and cleaned, but there was a slight problem. The engine fire had melted a small hole in the reservoir and totally destroyed the cap. The sensor was melted as well. I cleaned up the new master cylinder as best I could. Using a vise, I removed the reservoir and sensor from my leaky one to re-use.

The spring inside the sensor was rusty, likely from years of water in the system. Brake fluid is hygroscopic, meaning that it absorbs moisture from the air. This gradually lowers the boiling point of your brake fluid, which could be dangerous. That’s why it’s important to periodically replace your brake fluid.

Cleaning & Reassembly

To remove the rust from the spring and the mounting hardware, I soaked the parts in white vinegar for an hour. While I waited, I scrubbed the reservoir clean with soap and water inside and out. I also cleaned the area around the master cylinder. In addition to the regular dirt and grime you can expect, there was a lot of brake fluid on the brake booster that needed to be cleaned up.

After everything was clean and dry, I re-assembled the “new” master cylinder with the parts I was re-using from the old one. Getting the plastic reservoir on is no easy task. Lubricating the seals with brake fluid helped. If you’re doing this at home, it’s important not to use too much force, so that you don’t crack the reservoir. On the other hand, not enough force, and your reservoir won’t seal properly and leak.

I found it was easier to get the smaller front part sealed first. The seals rotate a bit, giving you some flexibility. Pushing against something soft — like the ground in your backyard — got the reservoir seated against the seals.

With the master cylinder clean and assembled, I installed it on the car. Once I add fluid and bleed the brakes, the car should leak a whole lot less — and be a whole lot safer to drive.

What’s Next

It seems like as good a time as any to completely overhaul the braking system and perform a few upgrades. Corvette Forum Vendor KNS Brakes graciously sent me a set of Hawk HPS performance brake pads to aid in the cause. These ferro-carbon pads offer an aggressive compound for high-performance street use and occasional autocross duty. Expect a full review in the next installment.

.