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warning valve[200/85] posted by robert guadamuz on
Friday, 4 June 1999, at 5:31 p.m.
I took the warning valve off and resested the light. The light stayed out for about 30 min. Then it returned. Does this mean a new valve. I also read that it might be the rear valves. There are two of them???? Robert
Re: brake failure warning valve[200/85] posted by Dave Stevens on
Friday, 4 June 1999, at 7:01 p.m.
The rear brake proportioning valves probably don't affect the switch that much unless they're leaking or blocked. I'd look elsewhere first. Further to the good advice you got from Don and Charles, there are a few DIY things you can consider:
If your switch was reset and now is back on then either
-the switch is faulty (contact/wiring shorting)
-the sensor mechanism is misbehaving (gummed up or leaking internally)
-there is a blockage well before the calipers
-there is a fluid leak somewhere in the system, or
-there is air in the brake system.
All of these present a safety issue -either directly or indirectly. I would not want to do a high speed panic stop in your car as it sits now.
First off, when you did the switch reset, did you see any signs of fluid on the tapered cylinder? It should be absolutely dry and clean in there. As it's often awkward to see in, push in a clean white rag and see what it picks up.
You should inspect everywhere for leaks which means maintaining maximum brake pressure while carefully inspecting each brake line connection, the master cylinder and the calipers. To properly inspect the calipers you will also need to unbolt them and swing them away from the rotors. Remove the piston dust covers and carefully extend each piston out about 1/4" (and not much more). A little dried reddish-brown fluid on the pistons is okay. Any serious grunge or visible fluid (especially on the inside of the dust cover) means it's caliper re-build/replacement time. While you're in there, clean them up with a toothbrush and little brake fluid. Rinse off all traces of brake fluid and grunge with spray brake cleaner. Replace any torn dust covers.
Follow your inspection with a thorough brake bleed to get rid of any residue and trapped air. Use unopened DOT 3 or preferably DOT 4 fluid only. It is very easy to introduce more air into the system during bleeding. The right equipment helps, as does a co-operative assistant, but technique is more important.
There should be all kinds of additional notes in the archives and the 740 FAQ brake section (the brake systems are very similar to the 240).
I'd then give the switch one last reset. Really stomp on it. If the warning light comes back on then you're probably talking switch block or master cylinder problems. But I'd rather see you off to the shop before you go much further.
Safe stopping -Dave
Re: brake failure warning valve[200/85] posted by Don Foster on
Friday, 4 June 1999, at 7:19 p.m.
Wow! Good list, good post, good info.
One more possibility -- sometimes, rarely, a flexible (rubber) line weakens and will balloon under pressure. Even though it delivers the pressure to the caliper, it consumes a volume of fluid -- and the differential switch detects this.
The "ballooning" may be as small as an 1/8" diameter increase -- you really need an eagle-eyed helper to find this problem. I've helped a friend chase this problem, and the easiest way is to grab the flex line (both hands) while someone stomps on the pedal. Feel for expansion.
Friday, 4 June 1999, at 11:20 p.m.
One of our kind folks posted a problem like this about
a week ago or so and it turned out that the rear seal on
his master cylinder was leaking into the booster. He had
bled and bled and bled but every time he DROVE the car
it happened. I suspect that the vacuum in the booster may
have helped kick it off, and siphoned some of the pressure
off the circuit served by the back part of the cylinder.
Hose collapse: I was experiencing this as early as 1973 or
1974 on a 69 Volvo 164. I think it is most prevalent in the
cars that have the tiny (1/8" ID) brake hoses. Took me a
LONG time to finally listen to the advisors that told me to
change hoses (with no further explanation).
George Downs Bartlesville, Oklahoma