BMW and MINI Brake Warning Light – Low Brake Fluid – Leak?
This answer is applicable for many BMWs in addition to the one listed below.
car year: 2003
car model: 325Xi
I was driving home and I noticed the red “Brake” brake light was on and then went off. Shortly thereafter I noticed a strong burning smell, different than my leaky valve cover gasket. I get home, check the owner’s manual and it says to check the brake fluid level. Sure enough it’s low. I figure it’s due to either a leaky brake line or the fact my rear brake pads are getting low. I’m surprised the fluid is low because this past July I had new front pads, rotors and calipers installed along with a fluid flush. I called my mechanic today and he asks if I have a manual transmission, which I do. He says it could be the brakes, lines or calipers but specifically because I have a manual transmission it could be the brake master or slave cylinder. Can you explain this and if it’s the master or slave cylinder am I looking at a huge repair bill?
The first step is to fill the brake fluid reservoir and see if the level goes down again, or you experience any further symptoms, such as; the clutch not disengaging properly. Additionally, you should perform a visual inspection of the general brake lines, hoses and calipers … as well as look for fluid draining from the transmission bell-housing (clutch slave cylinder leaking).
As for your questions about the clutch hydraulics:
The clutch master cylinder is under the dash, connected to the clutch pedal. When you push on the pedal this pushes the piston in the master cylinder and forces fluid through the hydraulic line to the slave cylinder, which is mounted to the side of the transmission. The fluid moving into the slave cylinder pushes the slave cylinder piston out and this moves the clutch release arm (which then releases the pressure plate from the clutch disc).
If the slave cylinder internal seal is leaking, this would leak fluid into the bell-housing (where the clutch and flywheel assembly is) and the fluid would then leak out of the forward bottom of the bell-housing, right behind the engine oil pan.
The master cylinder can fault in two ways; one would be to leak externally, like the slave cylinder. If the master cylinder is wet with brake fluid, it is faulty. The cylinder piston seal can also bypass fluid (due to weak sealing against the cylinder walls) without leaking externally. In this case, you would not lose fluid, but the clutch operation would be compromised.