Mountainrider

I have a 1989 L98 coupe. I noticed the other day that I have brake fluid leaking. It turned out to be coming from the pressure switch. It's that small white plastic switch thats on top and just a little to the side on the master cylinder. It has a wire going to it. So I whent down to my friendly Chevy dealer and bought one. Well I noticed today that the new one is leaking now. I took the old one a part to see if I could figure it out. Theres a plunger and spring inside and it looks like if the line pressure is to high on the front brakes it lifts the plunger and the fluid will leak out of the switch. So does anyone know if there is anything I can do to fix this or do I need to replace the master cylinder? I rebuilt the front brakes the other day and replace the rubber grommet thats under the front reservoir. I was thinking that maybe if I bleed the rear brakes that could help.

engle1147

There is a step in the plunger assembly...your hung up on it so the "new" part won't tighten down all the way. I had a similar problem with an '89 and had to take the entire MC off, remove the plunger & spring and install the "white" switch first....then reassemble the unit.
Hope this helps.

Mountainrider

Thanks so much I thought that I would have to do something like that. I wounder if I can lossen up the big nut that holds the proportioning valve and then install the pressure valve and tighten the nut back up.

Slalom4me

Bleeding the brakes MIGHT offer a solution.

The proportioning valve piston moves in the side cylinder to adjust
hydraulic pressure between front & rear. If there is a fault at one
end of the car, the piston moves far enough to activate the switch
and the cockpit warning light.

Perhaps some time spent at the wheel cylinders could pay off in
encouraging the proportioning valve to move back into the central
position without having to disassemble the master cylinder.

With the switch removed, can you determine which direction the
piston needs to move?

.

Slalom4me

Yes.

The catch is that air can be time-consuming to bleed out afterward.
The M/C is mounted on an incline, causing any air to rise to the forward
end of the prop valve chamber, away from the port.

Bench bleeding is the usual approach to address this.

However, an alternative approach is to have an assistant apply light
steady pedal pressure as you crack open the end cap on the prop
valve. The cap must be tightened again before the assistant releases
pressure or reaches the end of the pedal travel. If the air is only in
the top end of the prop valve chamber, this technique may help
evacuate it successfully.

Brake fluid is very hard on paint. If you go that route, have catch
trays in place and rags on hand. Wear eye protection.

.

Mountainrider

Fixed it
This is how I fixed it and it was free!!!
First these are all the tools you need. TOOLS: 14MM wrench,pick,15/16 sock brakerbar and brake clean.

Remove the end cap (its alum so use a socket only) Thats the proportioning valve part of the master cylinder.

Then remove the spring there was very little brake fluid in this part of the bore.

Then I used the pick to pull the proportioning valve piston out of its bore. Then I replaced the orings on the piston. When you install the piston with the new orings you will need to push the piston as far into the bore as you can. Then the switch can be installed.

Hope that this will help others save $21 that I spent for a valve that I didn't need to replace. The switch can be seen in this last photo its that white plastic part just off to the side. After you have moved the proportioning valve you can install the brake warning switch. If you look into the bore where the switch screws into as you move the piston you will see theres a grove in the piston. Aline that grove with the small hole in the bottem of the the switch bore and intsall the switch. Thats it Oh ya you do need to install the spring and cap too.

Slalom4me

Have the brakes been bled, yet?

.

coupeguy2001

mine was leaking, and i tried all sorts of things to stop the leak, but it turned out to be the piston cup was just worn out.
I even changed the plastic switch.
A new master fixed it.
The plastic isn't supposed to hold any pressure, just hold the spring in position.

Mountainrider

UP DATE I was having a problem with the brake fluid leaking out the warning switch. If you look in the photo posted showing me resetting the proportioning valve you well see the caps is removed and you can see the end of the piston. The piston has a sheetmetal cap on it. under that cap is a plastic piece of tubing that ifs into the end of the piston its about 1/4 inch long. inside the plactic piece is a metal dowl and in there is a rubber Oring and thats where the leak is caused. I changed that Oring and reset the piston and all is good in Corvette land once again.

Slalom4me

Did you find service replacement parts or use a conventional
industrial/jobber o-ring? If it is a jobber o-ring, is it butyl, nitrile,
silicone or ? ...

.

Mountainrider

I have orings that are from other brake jobs and fond some that if.

EDQ

Hey man, hope life fines you well. The solution you a describing fits my problem to a T. The photos would be very helpful but they are no longer showing on this forum, if you still have those pics of you working on the proportion valve in relation to the pressure switch alignment, could you please email them to me? Whether you do or don't, I would like to thank you for the info I received from just reading your posts.
harleydudebige@yahoo.com

Cliff Harris

If there is fluid in the port where the switch is located then there is an internal leak in the master cylinder. That port should be dry.