I recently had the brake failure light come on (84 308gts). The o-ring for the parking brake shaft on a rear caliper was leaking. Leak has been fixed and brakes work fine, but I can't get the light turned off. I tried opening both front bleeder screws, pumping pedal, hoping to nudge the sensor piston back. My car does not seem the have the type of pressure switch shown on another post with the removable plastic plate with 2 contacts, or I might not be seeing it right. (removing the wire from the shuttle valve does turn the light off, it's not fluid level) I would rather not remove the valve and have to mess with all of the brake lines.... Any great ideas? .....thanks....Larry Image Unavailable, Please Login
Larry, Having experienced this problem on a few F-cars, here's what I would do: -With an assistant holding constant pressure on the brake pedal, crack each line, individually at the brake master cylinder to ensure that there's no air bubbles trapped in the lines. As the fluid is under considerable pressure, just a slight crack of each line with a proper flare wrench will do the trick. -Repeat the same procedure at the differential pressure valve. Depending the on model of F-car, access can be poor. -Using a pressure bleeder, (the 308 cap is the same as used on virtually all BMW's, presuming it's a "screw-on" type cap), re-bleed all of the brake lines, starting at the LR, then RR, RF and lastly the LF. -If the warning light is still on, remove the differnential pressure switch and with a small screwdriver, or pick, center the internal piston so that the "notch" is centered. Reinstall the pressure switch and rebleed. Last resort: It is possible that, after many years of use and accumulated debris, that the piston is stuck in the differential pressure housing. If this is the case, the unit can be removed and rebuilt, as it only consists on an o-ring or two and a copper sealing ring. The end cap, however, can be a real bear to remove... Regards, David
If you decide to replace that unit, it is NLA from Ferrari. A similar part is used on the Volvo 240, it has a few extra holes but there are purpose made plugs available to seal those up. There are photos somewhere in the 308 Parts thread. http://www.ferrarichat.com/forum/showpost.php?p=136674143&postcount=6 Gerrit http://dino308gt4.com
The last time it happened to me, the solution was a poor contact on the pressure valve. See thisthread. I just washed the underside of the car (I should know better) & the wires got wet again and I have intermittent brake warning light. Time to clean the contacts again.
Also, did you bother to check the cap on the brake reservoir? There is a low fluid level sensor there that can also trigger that same light. Make sure it is properly connected, and that the float has not sunk. On my old Jaguars, the floats are cork and over time become saturated, and sink. But a bad connection or broken wire can cause the problem too.
Good point, Steve. It's far too easy to overlook the "obvious" and go down the hard road first...though, if I understood the orignal poster, he did disconnect the sensor lead at the pressure switch....and the light went "out". Regards, David
actually the light (safety switch) worked exactly as designed. RR caliper leaked while parked, first time I hit the brakes, light went on. fixed the leak, brakes work great, can't get the light out. only way the light will go out is unplug the wire at the safety switch (so it is not the res float switch). This is the part about Ferraris that totally annoys me.... switch works exactly as designed, with no information how to reset it. Maybe the 308 is mad because I bought another F-car......Larry
There was an great Q&A technical article on this in an old issue of Forza magazine penned by Tony Pallidino...probably in 2000 or 2001. Perhaps someone could scan and post the article.
??? Perhaps my post (#2) wasn't clear enough? If the differential pressure is zero, and the switch piston moves freely (internally), the piston will center automatically and the light will go off. If not,... "-If the warning light is still on, remove the differential pressure switch and with a small screwdriver, or pick, center the internal piston so that the "notch" is centered. Reinstall the pressure switch and rebleed. Last resort: It is possible that, after many years of use and accumulated debris, that the piston is stuck in the differential pressure housing. If this is the case, the unit can be removed and rebuilt, as it only consists on an o-ring or two and a copper sealing ring. The end cap, however, can be a real bear to remove..."
My picture in post 1 is from the Pallidino article, but as I said in my post, my later 308 seems to have a different unit....Larry Thanks for your reply David, I did understand the post, I don't understand why so much work should be required to reset the light. removing the unit on my car, where it is located, without damaging a brake line seems unlikely, unless I drill all the rivets out of the spare tire holder and remove it for access. it's rediculious.... I'm just frustrated....
Larry if you go under the front of the car and remove the access panel you will see the unit. I assume you have done this and have unplugged the switch. So now just pull the switch out of the unit. Stick an 1/8" drill inr the switch hole and move it around. Put the switch back in and plug it in. If the light is still on do the process again. You usually only have to move the internal valve a little to have the light go out. Also look at the bottom (contacts) of the switch and make sure they are not bent and touching. If so just spread them a little and reinstall. If you can't get the slide valve re-centered then you'll need to take it apart and rebuild it. After 20-30 years and some condensation the o-rings hang up and get stuck. So you can get these rebuilt at Whitepost or do it yourself.Good luck
Larry, I feel your pain. I had this problem with my BB about 10 years ago and had a very difficult time with the differential pressure valve, as well. Access was near non-existant, but possible with the correct tools. A trick, if you will, to get those brake lines loose, if need be, is to use a "Crow's foot flare wrench". They will fit on the end of a 3/8" extention and allow you to crack the lines free, as there's little room for a conventional flare wrench. If you're unclear as to the tools I'm referring to, just ask...and I'll post some pictures of what I use in the morning. This is exactly the problem. I recently rebuilt a NLA differential pressure valve on a 1976 BMW 2002. The bore was a tad rusty, with all sorts of crud on the o-rings. With a few new o-rings and a quick hone, she was as good as new... Regards, David
