735i Brake Master Cylinder
Summary
Replace failed brake master cylinder.
Start Date: July 16, 2012
Odometer: 309,000 km
Total cost: $350.00
After replacing my failed M50 engine, I took my car to a short track to test the installation. At the end of an hour-long session, the first 50% of travel on my brake pedal was extremely soft. The amount of braking on the track made me suspect boiled fluid and standard brake fade as the issue, but after completely replacing the fluid, the pedal was still soft. I decided to bleed the master cylinder to attempt to get all air out of the system, and discovered that no fluid was flowing out of the cylinder to the rear brake lines. The master cylinder had failed.
On E30s with M50 engine swaps like mine, the stock E30 master cylinder does not fit in the engine bay. The previous owner had installed a used E30 325ix master cylinder with an E30 325 ix brake booster. For this replacement I kept the 325ix brake booster, but opted for a new 735i master cylinder. The 735i (E32), which was also used in the E34 M5, is cheaper than the E30 325ix master cylinder, and has a large 25mm bore that adds to pedal stiffness.
Parts
The BMW dealer list price for the original Girling master cylinder was $606. Bavarian Autosport sells the same part for $179.95, but I was unable to wait for the shipping so I purchased the equivalent ATE part from a local garage supplier for $340.00. ATE is a manufacturer of some OEM BMW brake master cylinders.
Journal
Replacement went smoothly, and the result was fantastic. The pedal is now extremely firm, and on the track, firmer is better.
Here is the master cylinder installed. Notice its compact length. Click to enlarge.
Front view. Lots of room for the intake hose (not shown in the photo as it had been removed for the master cylinder installation). The two hoses leaving the top of the master cylinder run to the remote brake fluid reservoir, which is required to provide engine bay clearance.
Brake fluid melts paint and can damage for hoses, so I made sure to have lots of paper towel down to absorb spills. I also cleaned the driver's side of the engine bay with intake cleaner and a water hose after the procedure.