Re: 70 amx brake warning switch

[farna@xxxxxxx]

Darn it Tom, you made me pull my big auto text book down! Now it wasn't much help -- it didn't have a junction block with brake light switch explanation or illustration. But it did have a cut away of the combo valve. I checked a couple TSMs, and found a good cut away in the 1970 TSM. I have to admit to making a mistake -- the plunger DOES extend into the cavity the fluid passes through, but doesn't appear to cut it off. Thanks for correcting me on this! 

Combo valves, by the way, ARE NOT rather rare. They were used on all cars with disc brakes after 1970. There are two types -- one with proportioning (reduces rear brake pressure) and metering (holds pressure to front brakes off until pressure to rears reach ~100 psi -- what it takes to overcome the springs) as well as pressure differential (warning light) functions, and one with just proportioning and pressure differential. The first type was only used in the early 70s on AMCs, the second, simpler type is more common. The proportioning/metering/pressure diff type has both lines from the master cylinder to the combo valve, where one line then goes to the back, and two separate lines go to the front. The simpler type has a T in the line between the master cylinder and combo valve, with the T'd line going to the left front brake. A single line leaves the combo valve for the right front brake, so it's impossible to have a metering function, which would have to act on both front !
 brakes. 

--
Frank Swygert
Publisher, "American Independent 
Magazine" (AIM)
For all AMC enthusiasts
http://farna.home.att.net/AIM.html
(free download available!)

original message -----------------------------------------------------------------

Date: Wed, 11 Jan 2006 15:24:49 -0800 (PST)
From: Tom Jennings <tomj@xxxxxxx>
To: mail@xxxxxxxxxxxx
Subject: Re: 70 amx brake warning switch

On Wed, 11 Jan 2006 farna@xxxxxxx wrote:

> > To elaborate a bit, what it does is CUT fluid flow to the
> failed side, that's why it's relatively close to the master
> cylinder.


!! Are you SURE of that?! I'm not sure there's even a point to a
feature like that -- if you've already lost a lot of fluid,
chances are the lost-fluid side already won't brake. Second, if
it's job is to plug the low-pressure side to prevent more fluid
loss, then even if you then pumped the brakes the diff. press. sw.
would then be blocking the line!

I've also never heard of such a thing, though that could simply mean
I haven't been paying attention all this time.