Overview
The vacuum brake booster in my 1968 Mercedes 280SL was leaking air. Pressing the brake pedal made the engine run rough, and as far as I could tell, the booster had never worked properly. Replacing it with another original unit was getting expensive, so I started looking for a modern booster that runs on 12V and does not depend on engine vacuum.
The Bosch iBooster looked like a reasonable option. It measures brake pedal travel and uses an electric motor to apply force to the master cylinder. The pedal remains mechanically connected to the master cylinder if the booster loses power. I tested this after the installation: the pedal required a little more pressure, but I was still able to lock the tires without any problems.
Bosch made iBoosters for Tesla, Volvo, Honda, Volkswagen, Audi, Renault, Porsche and others. They are not all interchangeable. The bolt patterns, master cylinders and push rods vary, so it is worth measuring everything before ordering one.
I used a Gen2 iBooster from a Tesla Model Y. Its square 72mm bolt pattern is a near-perfect match for the T52 booster bracket on my car, and the booster slid into the firewall mount without modifying the car. I still needed an adapter plate for the original master cylinder, three spacers and a different clevis for the pedal linkage.
The first question was whether to keep the Tesla master cylinder. I flushed it and compared its fluid displacement with the W113 cylinder. The Tesla cylinder has a larger bore; I estimated that it would need about half the pedal travel and twice the pedal force to move the same amount of fluid without electrical assistance. That was not great for a system that should still stop the car if the booster loses power.
I decided to keep the original W113 master cylinder. It gave me the original pedal geometry and also meant that I did not have to disconnect the brake lines or bleed the brakes. The adapter plate mates the two parts.
The original master cylinder and brake lines stay connected during the swap, so there is no need to open or bleed the brake system. The adapter plate, spacers and pedal clevis use the existing mounting points without modifying any original parts. The iBooster can be removed and the vacuum booster reinstalled at any time.
The iBooster does not need a CAN bus for this installation. It needs a permanent 12V supply, ground, a switched 12V signal and four wires between the pedal travel sensor and the main connector. The wiring and brake pedal linkage pages cover those parts in more detail.
With all the parts ready, installing the booster was a weekend project. My used booster cost $175 and the complete conversion came to about $300 before tax and shipping. Used prices will vary. I found mine at Cali Motive; they also show up on eBay and at salvage yards.
After a brake test in the garage, I took the car for a test drive, made several hard stops and checked for leaks. The pedal felt responsive and I could not detect any lag from the booster. I do not have a useful comparison with a working vacuum booster because mine was already leaking, and I have not measured the stopping distance on a roller brake tester.
The original discussion and progress photos are in my Pagoda SL Group build thread.