Posted by Please Note: this doesn’t work unless you have someone who can make a new rod to connect between the pedal and the master cylinder, the stock one isn’t long enough. I gave up and decided to concentrate on something else, maybe I’ll revisit some day…
One of the areas of my car I’d identified as needing work after the end of the 2013 track day season was the brakes. Although the brakes aren’t short of stopping power, I’m not a fan of the initial bite or the overall pedal feel. One of the common complaints against the R53 is that the brake pedal is a bit lacking in feel and it certainly hasn’t been improved by fitting a pair of large AP Racing 4-pot brakes. A fairly common upgrade in BMW circles is to fit the larger diameter master cylinder from the M3 when upgrading the brakes on lesser 3-series models. I’ve read a number of posts on several Mini forums that claim the swap is possible but I’ve not actually seen a guide that explains how it can be achieved.
Rather than describe the entire swap (I’ll assume that if you are reading this you either have a workshop manual or a tame mechanic who can do the job for you, failing that you can read this excellent guide from Pelican Parts) I’ll just highlight where things aren’t straight forward, the bare master cylinder itself will just bolt straight onto the brake booster. If you are going to attempt the swap do yourself a favour and make sure you own a set of flare spanners, brake pipes are a pain to work with if you don’t have them!
- Fluid Reservoir: The fluid reservoir from the M3 won’t fit under the bonnet of the Mini, luckily the pipes on the bottom of the R53 reservoir has the same spacing as the original but the fittings on the bottom of the reservoir are different between the two cars. The solution is very simple remove the rubber grommets from your R53 master cylinder and replace the grommets in the M3 master cylinder with them. The R53 fluid reservoir will now fit onto the M3 master cylinder. If like me you got a master cylinder complete with all the ancillaries from eBay you might have noticed that the TORX screw that holds the reservoir on is fitted from the opposite side on the M3, this means you can’t just refit the old screw. The thread in the master cylinder is M5 and I decided to just modify a regular to secure the reservoir (you’ll need access to a lathe to turn down the screw or a lot of patience with some abrasive paper).
- DSC Pressure Sensors: The M3 master cylinder has two DSC pressure sensors one for the front brake circuit (marked with a V or Vorderseite – German for front) and one for the rear brake circuit (marked with an H or Hintern – German for rear) the DSC equipped R53s only have a single pressure sensor in the front circuit. If you have a car without DSC you can either block the sensors off or leave them in place. For cars with DSC you can remove the sensor for the rear brakes and block the hole. Although the sensors look the same the E46 DSC sensors will not communicate with the R53 DSC module so you’ll need to remove the sensor from your old master cylinder and put it on the M3 one. The sensors are in different positions (the R53 sensor is right on the bottom of the master cylinder) and the wiring will just about reach or did on my car. If you elect to leave the rear circuit sensor in place the plastic splash guard will need to be cut to clear it, I then patched the splash guard with fibreglass.
- Brake Pipes: Because the outlets from the master cylinder are in slightly different positions on the M3 master cylinder you’ll need to bend the front pipe to suit. This is pretty easy to do using a large ring spanner.
- Bleed ABS/DSC Pump: Since we are replacing the master cylinder it’s possible that some air may get trapped in the ABS/DSC pump which can be very stubborn to shift using the normal bleeding procedure. The ABS/DSC controller has a special mode which cycles the valves to push the air out. This can be triggered by a number of 3rd party diagnostic tools as well as BMWs INPA software.
Once the master cylinder has been fitted all you need to do is bleed the entire system, since you’ll have to flush all of the fluid out this is an ideal chance to put some high temperature fluid in (I’ve found Millers Oils Racing Brake Fluid 300 to be a cost effective alternative to some of the more exotic pure race fluids).
The E46 M3 master cylinder bolted into the car, the keen eyed might notice that the pressure sensor at the back isn’t the R53 type sensor this is how I know the M3 sensors don’t work!