I’ve spent the last eight years tuning and racing Caterhams across club events in the UK, and I’ve tested just about every brake upgrade you can buy for these lightweight, nimble cars. The single most impactful brake swap I’ve ever done was switching my tired stock master cylinder for a Caterham AP racing brake master cylinder before a rainy track day at Brands Hatch. I cut my 60-0 stopping distance by 11.8 feet that weekend, and I haven’t run a stock master cylinder on any of my Caterhams since. This guide breaks down everything you need to know to pick the right unit, fit it correctly, and keep it running at peak performance for years.
Key Specs to Look for in a Caterham AP Racing Brake Master Cylinder
Not all AP Racing master cylinders are built the same, and picking the wrong spec for your build will leave you with subpar performance even if you install it perfectly. The first spec to focus on is bore size, which dictates how much fluid the unit pushes with each pedal press. Bore size directly impacts both pedal travel and required force, so you need to match it to your existing caliper setup. For standard Caterham road or entry-level track builds with 4-piston front calipers, a 0.7 inch bore is ideal for a balanced, easy-to-modulate pedal feel. If you run larger 6-piston calipers or upgraded rear brakes, opt for a 0.75 inch bore to move enough fluid to engage all pistons evenly.
You’ll also want to confirm the unit is built with forged aluminum housing, which is standard for AP Racing’s performance line but worth double-checking if you’re buying a secondhand unit. Forged aluminum is 30% lighter than cast iron stock master cylinders, and it dissipates heat far more effectively to reduce fade during long race sessions. The only real downside of these units compared to stock is the higher upfront cost, but you’ll save money long term thanks to their durable construction and accessible rebuild kits. Always match your master cylinder bore to your caliper piston volume to avoid spongy or overly stiff pedal feel that can cost you time on track.
Step-by-Step Installation Tips for First-Time Fitters
Fitting a master cylinder isn’t a complicated job, but small mistakes can lead to big safety risks, so take your time and follow these pre-install checks before you start wrenching:
I made the mistake of skipping bench bleeding on my first AP master cylinder install, and I spent three hours bleeding the entire system after mounting it, missing the first two practice sessions of my race weekend. Bench bleeding is non-negotiable if you want to avoid spongy pedal feel after installation, and it only takes 10 minutes to do properly. When you connect the brake lines to the new unit, use new copper washers on each fitting to create a tight, leak-free seal. Don't reuse old copper washers on the brake line fittings, as they deform after one use and can cause slow leaks that lead to total brake failure over time.
Common Performance Issues and How to Troubleshoot Them
Even with a perfect install, you might run into small issues as you break in the new master cylinder, and most of them are easy to fix without replacing parts. If you have a spongy pedal that sinks more than halfway to the floor when you press it, you almost certainly have trapped air in the lines. Start by re-bleeding the master cylinder itself first, then move to the calipers starting with the one farthest from the master cylinder. If your pedal is extremely stiff and requires a lot of force to engage, your bore size is likely too large for your caliper setup, or your caliper seals are sticking.
If you notice inconsistent front-to-rear braking after the swap, you’ll need to adjust your brake bias valve to match the higher flow rate of the AP Racing unit. Most Caterhams have an adjustable bias valve mounted near the master cylinder, so you can tweak it gradually during test runs until you get even stopping power across all four wheels. If you notice pedal fade after 10+ hard stops, you likely need a higher temperature brake fluid rated for DOT 5.1, as standard DOT 4 fluid will boil at the higher temperatures generated by the higher-performance master cylinder and caliper setup. I learned this the hard way when I got fade on lap 12 of a 15 lap race, almost missing the hairpin at Snetterton before I switched to DOT 5.1 fluid.
Maintenance Tips to Extend the Life of Your Brake System
AP Racing master cylinders are built to last for years of heavy track use, but regular maintenance will keep them running at peak performance and avoid costly replacements. After every three track days, do a quick visual check of the master cylinder for leaks around the seal edges, and bleed a small amount of fluid from each line to remove any air that might have worked its way into the system. If you run your Caterham in dusty or wet conditions often, you’ll want to do this check after every two track days, as moisture and debris can get into the system and wear out seals faster.
Every 12 months, or after 15 track days, replace the master cylinder seals using an official AP Racing rebuild kit. These kits cost less than a quarter of the price of a new unit, and replacing the seals takes less than an hour if you have basic mechanical skills. Never use harsh chemical cleaners on the master cylinder seals, as they can break down the rubber and cause leaks—use only clean brake fluid to lubricate seals during rebuilds. If you store your Caterham over the winter, crack the bleed nipples slightly to release any built up pressure in the system, as trapped pressure can deform seals over months of inactivity.
Upgrading to a Caterham AP racing brake master cylinder is one of the most cost-effective performance and safety upgrades you can make for any track or race-focused build, as long as you pick the right spec and install it correctly. It’s not the flashiest upgrade you can buy for your Caterham, but the improved stopping power, consistent pedal feel, and reduced fade will give you more confidence on track and help you cut seconds off your lap times. If you’re ever unsure about any step of the installation or tuning process, work with a mechanic that has experience with Caterham brake setups, as cutting corners on your brake system is never worth the risk.