Thread: how to reset brake light valve

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I recently put an 10PSI RPV in the brake line coming off the master cyl, going into the brake light valve/ switch due to complications on using a disc/ drum master on a drum/drum system ( front brakes ither didn't work or came on after rears ), and my brake light was on, probably because of this, and it is still on, I know how the switch works, with the preassure needing to be equal otherwise the side with more preassure will overcome the side with less preassure forcing the ramp on the switch prod, closing the circut. How do I center the switch?

Originally posted by DennyW
Are you talking about a proportioning valve ? The type where if a line breaks, the valve moves to close that off, so you will still have brakes to get to the repair shop ?

If you are, you center it by opening the oposite bleeder, and that will allow it to move back to the center position, and put the light out.

yep, that's what I'm talking about, I'm not shure if mine is a preassure differential switch or a combination valve of sorts ( a true combination valve would have all nessary brake valving in which mine does not because both RPV's are supposto be built into the MC ) , which would be both switches in the same thing, but I'm guessing it's a proportioning valve also. Thanks for the help, do I just open the bleeder and have someone push it to the floor or do normal bleeding procedure and have preassure applied to the pedal b4 opening the bleeder?

Originally posted by Matt167
I recently put an 10PSI RPV in the brake line coming off the master cyl, going into the brake light valve/ switch due to complications on using a disc/ drum master on a drum/drum system ( front brakes ither didn't work or came on after rears ), and my brake light was on, probably because of this, and it is still on, I know how the switch works, with the preassure needing to be equal otherwise the side with more preassure will overcome the side with less preassure forcing the ramp on the switch prod, closing the circut. How do I center the switch?

what Denny said, but you need to know which wheels the valve has cut off and bleed that end. have some one set in the car, key on, and push the brake paddle down as you bleed it. they should watch the light and when the light goes out don't push the paddle no more and you close the bleeder. if the paddle goes all the way down and the light don't go out, go to the other end and do the same thing. If the person pushing the paddle down don't stop when the light goes out then it will trip to the other wheels. this is on a ford, GM center themselves.

sounds good, thanks guys, I think it's the rears, I bled the fronts and wasn't getting any signs of fluid restriction at the bleeder. I'll probably do this tomorrow night.

Using a disc/drum master cylinder on a drum/drum system is dangerous. The disc side moves more fluid volume than is needed to properly activate the drums.....regardless of the residual valve. This may be why you get the unbalance signal

From the 911Brakes web site:

MASTER TYPE DESCRIPTION
DRUM / DRUM A drum / drum master is designed to deliver fluid pressure and volume to the front and the rear of a braking system in equal proportions. Drum brakes will require less fluid and pressure than disc brakes. Typically a drum brake master will be smaller than a disc master and the fluid reservoir chambers will be equal in size. Since drum brakes require the use of residual pressure valves the original drum master cylinders had residual valves built into the outlets. Later model aftermarket units do not have these valves and they must be installed in the lines externally. Failure to incorporate residual valves will cause spongy brakes.

Generally speaking it's not a good idea to use a drum brake master for disc brakes since the amount of fluid the cylinder pushes will not be sufficient.

DISC / DRUM A disc / drum master is designed to push more fluid volume to the front disc brakes since disc brakes require more volume than drum brakes. Generally speaking a disc / drum master will have one fluid reservoir larger than the other.This is because the disc brake pads wear faster than the drum shoes and the fluid reservoir will drop faster. Original disc / drum masters had a built in residual pressure valve to the rear drum brakes only.

Never reverse the outlets on a disc / drum master. Doing this will cause the front disc brakes to drag excessively from the residual pressure valve.

Originally posted by robot
Using a disc/drum master cylinder on a drum/drum system is dangerous. The disc side moves more fluid volume than is needed to properly activate the drums.....regardless of the residual valve. This may be why you get the unbalance signal

From the 911Brakes web site:

MASTER TYPE DESCRIPTION
DRUM / DRUM A drum / drum master is designed to deliver fluid pressure and volume to the front and the rear of a braking system in equal proportions. Drum brakes will require less fluid and pressure than disc brakes. Typically a drum brake master will be smaller than a disc master and the fluid reservoir chambers will be equal in size. Since drum brakes require the use of residual pressure valves the original drum master cylinders had residual valves built into the outlets. Later model aftermarket units do not have these valves and they must be installed in the lines externally. Failure to incorporate residual valves will cause spongy brakes.

Generally speaking it's not a good idea to use a drum brake master for disc brakes since the amount of fluid the cylinder pushes will not be sufficient.

DISC / DRUM A disc / drum master is designed to push more fluid volume to the front disc brakes since disc brakes require more volume than drum brakes. Generally speaking a disc / drum master will have one fluid reservoir larger than the other.This is because the disc brake pads wear faster than the drum shoes and the fluid reservoir will drop faster. Original disc / drum masters had a built in residual pressure valve to the rear drum brakes only.

Never reverse the outlets on a disc / drum master. Doing this will cause the front disc brakes to drag excessively from the residual pressure valve.

I'll be changing to disc front brakes in the future ( granada swap probably, then I'll replace the old RPV with a 2psi RPV designated for discs). It will make the front brakes lock a little quicker, but when you do the math in relation to pascals law, if you put 50 PSI of preassure on the pedal and move 2", than there will be 100 PSI exzerted on the master cyl ( 1" bore, make it easy ) , which then if it goes to a caliper, the pistion ( say 3" ) will exzert 300 PSI but only move 1/3", in a wheel cyl ( 1" bore ), will exzert 100 pounds, and move 1/2" ( in my auto class, I'm learning brakes now, so I have to know all this stuff ) . they were coming on after the rears locked up b4 I put the RPV in. The bore size in my new master is the same as the old. Thanks for the info.