Please help me solve a problem with my 2001 Prius with 153k. For some time the ABS brake light would mysteriously come on, and usually go away when the car is turned off. The dealership could not find any codes and released it for further "monitoring". Then the brake pedal took to sinking an inch when the pedal is held firmly down. Now it even drops an additional inch again if you continue to hold it. The ABS doesn't seem to be functioning now. The dealership found this code recently: C1246. They say that it is for "Master Cylinder Pressure Sensor" and recommended replacing the master cylinder since they can not obtain the pressure sensor seperately. An independent shop felt it was the sensor in the bottom of the brake master cylinder fluid reservoir. So I ordered in a used one (low mileage) and had it installed. Problem did not go away. Do I need to buy another OEM master brake cylinder? Please advise.
This is a very tough problem to advise you on from a forum. Here's what I can say: Obviously you have brake problems. Brake problems are not to be messed with (ie: brake failures kill or cause accidents). If I was in your shoes, I would take it to a brake specialist and see what they say. Master cylinder's are really low failure parts meaning they rarely go bad. But, they do on occasion go bad. Obviously the Prius regen brakes are very complicated. It is one of the least understood systems on the Prius but, one of the most critical. It is my guess that replacing your master cylinder will not correct your problem but, that is just my guess. I'm no expert.
I never know quite what to say when a mechanic feels that such-and-such is the problem. Maybe I'm just getting caught up in words but, you know, it either is the problem or it isn't, and if it isn't, something else is, and it would be great to find out what the problem is so it can be fixed. In Volume I of the shop manual under C1246 there won't just be a terse little description like "master cylinder pressure sensor," there will be a reference to the page number where a whole diagnostic guide and decision tree for that code begins. If I have time in the next couple days I can look it up in mine, and if the possibilities are simple enough I might be able to copy them out for you (or somebody else with access to the manual can). If you're not that patient, it's easy to get access to the manuals yourself at techinfo.toyota.com for a cheap subscription (the shortest is two days for fifteen dollars). If you (or your mechanic) can go through the basic, published diagnostic steps for the problem, either that should lead directly to pinpointing the problem, or you will have gathered more information to post here that should help us suggest useful next steps. -Chap
